Ah yes, the age old question- if only it was that simple. I'm going to give two answers to this question, the 'rational' answer, and *my* answer. 

I'll go ahead and give you the rational answer now. Neither system is better, they both have pros and cons, and it depends on the needs of the photographer which one would be better suited for them. This is the answer you'll probably read everywhere else. You can go ahead and stop reading here and leave this page, and move on with life now. Oh? You want to hear my opinionated answer based on years of experience with both systems? Ok...alright, fine ;]


I'm a Nikon shooter, but I prefer Canon. Here are some reasons why. 

To start you off with some background, I started out with Canon camera equipment, and stayed with it for about four years. I've owned most of everything in the professional lineup (at least that's relevant to wedding photographers) and can directly compare certain lenses from both brands. When all of the hyped up new Nikon cameras started coming out with gobs of dynamic range, along with reasons I will give later, I decided to jump ship to Nikon, which I've been shooting for a little over a year and a half. The grass is always greener on the other side, and well, in this case the color tint is greener. 

To simplify this post, I'm going to break some of the pros and cons that make or break both brands for me personally, so that you can skip over to what you think might be relevant to you.

Lenses

This is a hard section to cover because lenses are truly subjective. Some people prefer the rendering of Nikon lenses, and some will prefer Canon's. I'm going to skim over the offerings that are relevant to wedding photography. 

1) 70-200

Canon has this gem called the 70-200 2.8L IS II. It is an absolutely stellar lens. It's sharp, the stabilization is excellent, it's built like a tank, and it's white, which consequently made my hands sweat less outside because it didn't get as hot. This lens engages IS super fast and the compression is unreal. Fact: you can get way more compression from this lens than the Nikon counterpart, because of something called focus breathing. 

The Nikon 70-200 2.8g VR II is no slouch, it's very sharp and has nice, smooth bokeh. It lacks in two areas though, in my opinion. First, the VR, though rated the same as Canons (4 stops I believe), is nowhere near as good. It takes longer to engage, and I get way more shots with camera shake in them than I did with the Canon. Second, the focus breathing is attrocious. 

If you are zoomed in to 200mm with the Nikon, at headshot distance, the lens is effectively only about 135mm. The Canon on the other hand, focus breathes in the opposite direction, giving an effective length of about 226mm. This means two things; you can frame much more tightly with the Canon version which makes it more useful for detail and close up shots, and it is literally impossible to get the same degree of compression from the Nikon. 

To be fair, the Nikon focus breathing eases up near infinity, so far long range reach they won't be too different. But if you're looking for compressed portraits, headshots, or group shots, the Canon gives a look that's impossible to get with the Nikon version. (the Tamron also focus breathes badly, just throwing that out there).

2) 24-70 2.8

 

I hate this lens, and use it only out of necessity. It's not a pretty lens, it doesnt give awesome bokeh, it's just a boring lens. 

But, for dramatic shots using off camera flash, it's great for working quickly, and for certain parts of wedding receptions like entrances and action shots, it's really convenient to have. 

I've had the Canon V1 and V2 of this lens, and the Nikon version currently. Canon's v1 was decent except the mechanical parts wore out every year which require servicing because it elements would decenter and the image quality would turn to crap. The V2 of canon's 24-70 fixed that issue, was one of the sharpest lenses I've ever seen in my life, and was lighter. There was also less distortion at 24mm. Canon's 24-70 2.8 ii is the best 24-70 that currently exists, period. 

Nikon's 24-70 2.8g is a good lens too, though. It's nice and sharp and does the job. If you want to get really picky, the Canon v2 is noticeably sharper, especially in the corners of the frame, and the Nikon has some really bad distortion at 24mm. 

3) Top shelf 85mm primes

Canons 85mm 1.2L ii is my favorite lens of all time. It's sharp enough to use at 1.2 whenever you want, and has a melt away bokeh and depth of field that's so shallow that some times it looks almost tilt shift like (especially on automotive photos). The focus is super slow as many of you know, but it's very accurate. As long as you're not trying to go from MFD to infitiy, it's fine. I've tracked children running with it, and nailed nearly all the shots in the sequence. 

Nikons 85mm 1.4g is also a beautiful lens. It's also sharp enough to use it wide open whenever you want. The bokeh rendering is completely different though, which is hard to describe. I also had to go through three brand new copies and then sending my current one in twice because of focus issues, while the canon was perfect. All three copies I got of the NIkon need AF fine tune values ranging from +12 to +20, and it depended on your subject distance, so you couldn't just set one and forget it. A few trips to nikon fixed the problem, but it was a problem that shouldn't have existed especially on three separate copies that were brand new. Did I mention it took multiple trips to service to fix it? More on that later.


4) I'm going to stop with the lenses here, as for 35 and 50mm primes I have used the sigma art series for both brands. I have briefly used the 50mm 1.2L and it was lovely, and also have the Nikon 58mm 1.4g which I have to say is one of my favorite lenses. Nikon created a very unique lens with that one. 

Camera bodies and functions that are important [to me]

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Obviously both brands make a wide range of cameras. For simplicity's sake, assume I'm referring to the full frame semi-pro and pro models.

 

I extensively shot Canon 5d mark iii and 6d bodies, and currently use two Nikon D750's. Functions I refer to are relevant to the current lines of cameras from both brands. 

1) Autofocus

When Canon released the 5d mark iii, we were all thrilled because the awful prehistoric focus diamond was finally gone. Prior to the 5d mark iii, you had to buy a 1D series body to have a high end focus system on full frame with Canon. The Canon 6D was definitely better than the 5d ii, but it still had one cross type focus point in the center, and the rest are kind of a gamble. 

Current high end full frame cameras from Canon, including the 5d mark iii, 5ds/r, 1dx and 1dx mark ii have a similar looking system. There are dual diagonal focus points in a center column, which can pretty much focus on anything with contrast in any direction at critically wide apertures. Nikon does not have dual diagonal points on any camera. The same system also has many cross type focus points on the sides of the frame, which allows you to freely use whatever points you want without worry too much about reliablity. These points do a great job of grabbing focus, especially in dimmer light. There's also a function called spot focus, which allows you to reduce the size of the focus point to really pinpoint focus. I didn't realize how helpful that was, until I didn't have it anymore. 

One of the things that surprised me about Nikon is that they did not have a single camera with cross type focus points on the left and right sides of their focusing systems, and only had them in the center cluster (Except the D5 and D500 which are brand new, but they'll cost you a lot of money). I assumed this was because they had some awesome autofocus and it would be just fine. 

No.

While outdoors in bright light the side AF points are decent, indoors at wedding receptions they are very hit or miss. I've found the need to rely on my old friend, focus and recompose, more than I should have to. Even the D750 with its praised focusing system struggles to nail shots in average to dim wedding reception or ceremony conditions, if using side AF points which are linear. Sadly, there are some situations where even the off center points on the Canon 6d have given me better results. The 5d mark iii could nail nearly every shot with any point at f1.2. I have seen so many praises for the D750 or other Nikon camera's autofocus systems, but am starting to wonder if they're even using anything other than the center focus point. My experience with the D750 has been absolutely awful as far as autofocus goes. 

Another function related to autofocus is that Canon allows you to leave your camera in 'single shot' AF mode, and program a function button on the front that when held down the camera will switch to servo focusing. This way you can very easily and quickly switch between the two modes when needed. It takes more time to switch with nikon, so most people back button focus and leave the camera in continuous focus mode all the time. It's important to note that when doing this, the camera does not actully lock focus and stay locked, so any minor twich of your hand can cause the camera to start focusing again when you don't want it to. 


Custom setting banks 


This is the most under used feature I've seen. When I talk about this and why it's a big deal to me, most of my friends give me a blank stare and tell me they don't know what I'm talking about. 

I'm referring to Canon's C1, C2, and C3 bank modes, and Nikon's U1 and U2 banks which are similar. 

Essentially, you can save all of your camera's settings into a bank to use them later. This includes ALL of your settings, not just the exposure triangle. 

What I found extremely useful was to first save my default settings to each bank (camera set to raw, autofocus settings, functions, etc. ). That way nothing I didn't want changed, wouldn't get changed. Both Canon and Nikon do this. But Nikon lacks one very, very important feature: being able to enable auto recall of last used settings. 

When you save a particular set of settings to U1 or U2, they stay that way until you save new ones by going through a bunch of camera menues. As soon as you leave that U1 or U2 mode to use anything else, the settings revert to the ones that had been saved. 

Canon's system allows you to have the camera automatically save any tweaks you make to settings while in a C1, C2, or C3 bank, so that when you switch between then or other modes, you can go back and it's all still there like you left it. 

Why do we care? We don't use this anyways.

Well you should. This effectively gives you three separate manual settings to switch between. Here are some examples of how awesome this is:


You're shooting down the aisle, the altar is backlit. Shooting from the front down towards the back, it's direct sun, and shooting from the sides it's a mix. 

You dial in your exposure for each direction. When you change directions, you don't have to shuffle settings, you just move your mode dial by one click and they're perfect. You could use aperture priority, but it's going to give you inconsistent exposures due to back lighting. This is the easiest way to nail consistent and perfect exposures with different lighting situations. When you need to tweak settings (say, the clouds are rolling in), you can and they stick, instead of getting wiped like when you leave a Nikon U bank. 

Or, you could have a wedding that's partially inside, partially outside. Put your settings for inside on C1. Put your settings for outside on C2. You walk in and out of the venue with confidence knowing all you have to do is make one click over on the mode dial. 

Or, you're shooting natural light portraits, but want to throw in some off camera flash shots here and there. You have your natural light exposure on C1 and your dramatic flash shot exposure on C2. Working has never been so easy. 

In summary

These are just my personal opinions and features that make me prefer Canon over Nikon. I had gotten so used to having certain simple features that I didn't realize they weren't on all cameras. Ultimately, for me, good autofocus with cross type sensors on the sides and spot focusing, auto recal banks, instant autofocus mode switching, melt away bokeh, and more vibrant colors in raw files for fast workflow are more important to me than the extra dynamic range I gained by going with Nikon. I forgot to mention customer service and quality control, but, I should probably not, because I could write twice as much on that ;].