I'm shooting with my 5DMK III and my 7D. I really don't find my 5D3 AF any faster than my 7D with it's f/2.8 lenses... Not when I'm shooting outdoors anyway. I do a lot of BIF and still find myself reaching for my 7d with the 100-400mm before I reach for the 5d3. In fact, I seem to have more AF issues shooting static outdoors in darker situations with the 5d3 and the 24-105mm f/4.0 than I do with the 7d paired with the 17-55mm f/2.8.
I wanted the 5d3 mainly for portraiture, wedding and landscape work, although I've had no issues with my 7d and landscape work, I have had with portraiture and weddings. My 7d is consistent with regard to high noise in the shadows in most of the portrait work I do, resulting in considerable time in post to resolve it effectively. Having numerous requests for evening weddings and then outdoors on top of that has created a very frustrating experience with my 7d and those types of shoots. The 5d3 is a welcomed change for me regarding those very frustrating low light portrait or wedding shoots. Coupled with the 85mm it's wonderful, 70-200mm f/2.8L IS II, it's engaging and much more fun!
I really love having both cameras. I've been complaining about my 7d for night or indoor weddings and other events for some time along with the noise issues even in some outdoor portrait work. The 5d3 is wonderful for that.
I still tend to reach for my 7d in birding and wildlife. It seldom misses the mark and on a recent outdoor wildlife shoot with both, the 7d was bringing home the winners over the 5d when the reach was needed, even being able to crop the ff images more. I find the 6 fps quite different over my 7d's 8fps. I didn't think I would but it's definitely different. Not quite as satisfying for me... When I bought the 5d3, I did so with every intent on selling my 7D, alas it still resides with me and in it's bag with the 5d3 now residing comfortably beside it in the same bag... I am considering some new teleconverters to use with the 5d3 though, that may be all I need to go FF only...
In the end, I'm looking forward to seeing what Canon does with a 7D MK II. If it's FF, so be it. I'm positive it will be awesome whether it's crop or FF. I've often thought a FF 7D would make a very useful and flexible option to cover many areas of photography, rather than just sports and wildlife... It may be a very well rounded option...
