But buying a 6D is trading AF performance with fast-moving subjects for ISO performance. Personally, I'll take an in-focus, noisy image over an out-of-focus, low-noise image.
Personally, I wouldn't want either :-p ... the questions are:
1. Is the shot is repeatable or mission-critical? If it's not that important to nail this exact shot, you can try again or shoot multiple frames with re-focusing with the 6d and get a good looking result sooner or later. With a 7d/70d crop, maybe you won't have to try again, but it'll be 100% crappy iq starting with iso1600+.
2. Is multi-point af tracking required or desired? If using center-point only which has been a working system for decades, the otherwise very underwhelming 6d can be a good alternative even with the outdated af system unless using a very fast lens.
Agree, although the AF tracking with all points active on the 6D does work. It's just that much of the time, mine neglects locking with center point for some reason in this mode. It prefers the slower, less reliable outer points. If it just wouldn't avoid the center point with all points active as often as it does, it would do better.