Hi Wills, I am enjoying my 6D too. The high iso ability makes the impossible possible or the hard easy. I really had to work with the 7D and the primes to get shots that I can get easily with the 6D.
The center focus spot is really good too.
I choose the 24-70 Tamron 2.8 VC to go with the 6D, mainly for the portrait, ability and its actually a useful lens on the 7D. Often we would have the 70-300L on the 7D and the Tamron on the 6D and then swap the lenses over. The 6D with the 70-300 is lovely and so is the 7D with the 24-70 2.8 , its benefits from the 2.8 aperture.
I did a 2month trip with the family and caravan last year and bought the 17-40L just before the trip. Its a lovely match to the 6D, weight and balance wise. The 24-70 can be a bit long thick and heavy. IQ has been fine, but would suggest you shoot in raw and use things like LR4 to fix vignetting. I personally was using it for landscape and general photography, so coupled with the high iso ability I don`t see the point in the 2.8 aperture, adds too much weight, size and price. The slightly better IQ might make it worthwhile, but like I said using a good processing tool and I don`t find the step up worth it. But since I don`t have it I don`t know what I am missing. Having the 24-70 2.8 to fall back on is obviously a bonus.
Basically on most days when travelling, I had the 17-40L on the 6D and the 7D with the bigger 70-300L, that is what I take bushwalking. Around at home etc the 24-70 lives on the 6D.
I dismissed the Tokina because its as big and heavy as the canon 16-35 F2.8L canon and reviews suggest that the 17-40L is better
Canon Powershot S95, Canon 6D,7D, Canon 40 2.8 STM, Tamron 24-70 2.8 VC, Canon 17-40 L, Canon 15-85, Canon 85 1.8, Sigma 30 1.4, 50mm 1.8, Canon 100 2.8L Macro, Canon 70-300L +Kenko 1.4 Pro 300DGX, Canon 430EX II and RS 4 Classic