First off, I'm a Canon guy. Nothing at all against Nikon. If I could afford to own a full kit of both brands I would, but at the costs of lenses and keeping everything maintained and up to date that would just be rediculous. Both have stronger and weaker aspects, obviously neither is the clear superior. They even leapfrog each other periodically in certain aspects, which keeps the competition up to always improve. In fact, before the latest fleet of cameras from Canon were released I was thinking about switcing to Nikon, but thankfully, my new camera (read on) is (in my opinion) the finest camera for wedding photographers ever made. Your move Nikon.
I buy all my equipment from B&H Photo. Although they have a zillion (Hebrew) holidays throughout the year that they are closed during, they generally have the largest selection, consistently good prices, and a healthy amount of reviews about most products.
This is my main camera body. I love it!!! It's a 22.3 megapixel full frame camera. In this day and age, high resolution isn't as important for event photography as is the ability to quickly lock focus and get clean, crisp images in very low light. It has dual card slots, which is a great feature! With as many photos as I shoot, my memory cards get filled and emptied very often and this raises the possibilty of one failing. Having photos saved to two cards at once all but elliminates the possibility of losing a critical photo in the middle of a wedding or other photo shoot. I also have the option to have one card serve as an "rollover card" so that if I happen to fill the main card at an inopportune moment I won't have to stop shooting.
The battery in the camera is rated to about 1,000 shots per charge, and I go way over that, so I have the battery grip that holds two batteries, plus gives you the ability to shoot vertical shots without using a different hand position since duplicate controls are on the side of the grip. I find this helps me keep vertical shots lined up better since I don't have to reach around awkwardly.
The MK3 has a very fast and accurate autofocus system and it works even in the very dim lighting situations that come up all the time during a wedding. It also maintains very clean images even at very high ISO so I don't have to sacrifice image quality for a properly exposed photo.
I shoot in RAW, and if you're a pro shooter, you absolutely have to. There's some shooters out there that insist that if you "get it in camera" in other words, use the best possible camera settings and/or lighting techniques, you don't need much further editing, but it's important to understand that the human eye is much much more capable than even the best camera sensor, and the amount of luminocity and tone that can be captured in a single photo is much lower than what we see with our naked eyes. Even if you take the "perfect shot" there is always something there that can be improved with editing, and shooting RAW gives you almost infinitely more flexibility in this regard. I won't explain the whole process, but think of it in these terms. If you had to write a song, would you want to pick 8 keys of a piano first and then write a whole song using only those keys, or would you just want to keep the whole piano? You probably will not end up needing all 88 keys, but having those extra 10 octave will open up so many more options for you.
I use full manual settings (M on the contol dial) 99.999% of the time. Even a top of the line camera like this isn't quite to the point that it can make decisions as well as I can with its Auto, "P for Professional" and priority modes..
EVERY professional photographer should have a backup camera. When you're shooting thousands of images at a time on a weekly basis, even a high end camera has a possibility of having its shutter break or some other error develop at any time. Furthermore, I don't believe that your backup camera should be the Rebel or whatever consumer grade camera you just upgraded from if you're new in the market. If your camera goes out in the first hour of wedding coverage, your client doesn't deserve subpar photos because your backup gear couldn't stand up to the challenges of the wedding day.
My backup camera is the 5D Mark II (previously my primary body). Yes, it's a generation older than the MKIII, but tons of top end wedding pros still use this body and it's capable of brilliant image quality. I've shot many great weddings with my MKII and if my MKIII were to bite the dust, I'd be bummed, but I wouldn't be the least bit nervous about whether or not I could continue to deliver the quality of images my client hired me for.
I have the time signatures of my cameras synced together so that if I happen to use both bodies at once, or have to switch to the backup my photos will sort chronologically during editing without any problems.