Quote Originally Posted by gbc View Post
Plan on keeping my 7D, both to have a backup body, and for the extra reach for wildlife and other far-away shots. Or would a cropped full-frame shot at 300mm still be better than a 7D/300mm shot?
A 6D shot cropped to the APS-C FoV will have equivalent IQ to the upcropped 7D at low ISO (less that ~800), and better IQ at higher ISOs. So, assuming 7.8 MP is enough (prints up to 16x24"), there's no advantage to the 7D from an IQ standpoint.

However, the AF of the 7D is better for moving subjects. If you were getting a 5DIII, I'd say the only use for the 7D would be as a backup (I'm not even sure I'll keep my 7D, after getting the 1D X I haven't even used it for anything but some test shots). But with the 6D, the 7D is going to be better for moving subjects, and I think better AF will trump IQ in that scenario, at least up to about ISO 3200 - above that, the 7D IQ is not really usable, IMO, and if the 6D's AF can't keep up you'll have hit the limit which only the 5DIII or 1D X would let you overcome.