I current use just one 5D Mk III, and I am looking to get a second body. I'm starting to get annoying with having to change lenses constantly during an event/wedding. I also am not liking the idea of relying on just one body. It would be nice to have my 24-70 or 16-35 on my main body, and my 70-200 on my second.
My options are:
Buy a second factory refurb 5D Mk III after I receive my tax returns at the first of the year.
Buy a 6D now.
Or just wait for the rumoured 5D Mk IV to come out on early 2015. (least likely as Canon likes to push back releases)
I can not see myself using a crop sensor camera again, I guess I'm spoiled.
Well, if you're doing videos, I'd stick with the same body. It'll make it easier to maintain consistency in the video, resulting in less time spent in post and a more streamlined workflow. If it's just stills, then a different body would be fine since consistency isn't as important, nor as noticable. So, if you're doing video, go Mk III. Stills, 6D (and if you decide you want an upgrade later, sell it and grab a Mk IV whenever it comes out).
Just my 2 cents.
If your doing event photography and receiving money for it you absolutely need a second camera. I shot a wedding recently (Stills only) with my 5Dmk3 and a 7D. The 7D was used for wide shots (Used the Canon 17-55mm) and used the 5Dmk3 for the 70-200mm and 85mm with a little 14mm. If I were in your position I would get the 6D and glue the 16-35mm on the end of it (maybe switch the 24-70 if your feeling good). Think of a 6D as a 5dmk3 with only one auto focus point (although its a really good point). Since a wide angle lens doesn't require a large amount of work from an auto focus system since the DOF is much larger I would keep the wider angles on the 6D. This would work very well. Then dedicate the 5Dmk3 to the longer 70-200mm and your fast primes.
The only downside to the 6D is you lose that precious joystick. But since its only one usable auto focus point the joystick isn't as important. The ergonomics is slightly different from body to body some there is some learning there. Once you do it you will get use to it and will have a great setup
Nikon user here, so I don't know much about Canon's new offerings. But on a professional standpoint, yes, you do need a second body. Or even a third one as backup, if you can. You'll never know when your main will crash unexpectedly (I hope not).