neuroanatomist said:
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

... 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has been a working system for decades, the otherwise very underwhelming 6d can be a good alternative even with the outdated af system unless using a very fast lens.