All Canon bodies will support autofocus with all available focus points when used with f/5.6 lenses or faster. Thus, you would have the use of all AF points using the 600mm f/4 plus a 1.4x teleconverter with either the 7D or the 70D. For lenses slower than f/5.6 (e.g. 800mm f/5.6 + 1.4x), AF is only available on the 1D pro bodies or the 5D Mk III with the latest firmware but AF support is limited to the center focus point. With respect to your lens choice question, I would suggest you take a look at this :
http://www.birdsasart-blog.com/2013/...e-winner-is-2/