Alan Bryant , Oct 26, 2012; 11:08 a.m.
The 6D isn't available for testing, so nobody can say for sure.
I've used the 5D1 and 5D3. Based on testing I've seen, the 5D3 and 5D2 have very similar image quality. They both have Live View which should help a lot with focusing tilt-shift lenses. I'd suggest you go with a 5D2, which is really a bargain right now; or wait for the 6D to be available.