Advice on purchase of 6D
I recently bought a 6D coming from a 20D. I have the 24-105 mm. The quality of the 24-105 on the 6D is fine but distortion in particular at 24mm and 105mm is significant. I still prefer to use my primes, 50mm 1.4 , 150 mm 2.8 ( Sigma) a lot. The low depth of field, nice bokeh, 2 or 1 stops of light extra and absence of distortion of the primes is still a real bonus.
If you are used to primes and zooming by feet, I would buy the 6D body and stick to primes. It is IMO the best combination possible. The image quality of the 6D blows you away at high ISO! I recently used my 50mm at a ballroom dance evening and I ended up using the 50mm at 1.4 without flash at ISO 12800 or 25600. The AF was significantly faster with the 50mm than with the zoom. Below F2.8 the centre AF point is extra sensitive.
Good luck with your decision!