I can't comment on the new 6D because I haven't seen, one but I sure am impressed with the low light AF performance of my 2 5D MK lll bodies. Mine work extremely well in very dark conditions. I use a 24-70, 16-32, 70-200, 300 2.8 and 300 f4
Camera selection in my opinion is usually based many features and specs. If the 6D focuses better in low light that's really great.
We want our cameras to hold value, and I think most of us want new models to feature improvements. The reason I began following CanonRumors.com was to be informed of new equipment on the way so I can make smart purchasing decisions. I imagine the 7D will focus better than the 6D. Its really only a problem if you buy the 6D a month or two before the new one comes out.