What I find almost impossible to gauge from all the informed opinion and debate about ISO charts, sensor IQ and the like is just how much of real-world difference I’d notice and would like the opinions of anyone with greater knowledge than me. To be clear, that’s pretty much anyone who regularly remembers to take the lens cap off for their first shot .
If you attached a Canon 85 1.2 L to both camera's, T2i and 70D, and took a photo, I doubt you would see much difference, unless the 85 needed to be micro-adjusted or it would not acquire focus because of the single type outer AF points that the T2i has. High ISO JPEG images would look much better on the 70D as well.
I have owned and used all the camera's in question and my advice is to purchase the 70D and not the 6D or the over-rated 7D; you will be much happier. I now own the 70D and did own/use the 7D for around eleven months, and bought and used the 6D for about two months.
The 7D does not focus very consistently; for my 7D, AI Servo yielded around a 60% hit rate for indoor/outdoor sports. And the IQ was not that great on these RAW images either, a bit soft even when focus was spot on. I then bought a 1D3, and my hit rate went to over 90% and the 1D3's images were so much sharper than the 7D's images. Goodbye 7D and have never missed it.
If action/fast moving objects are important for you to be able to capture, then forget the 6D. It is an incredible camera if you understand it's one limitation, the outer AF points. Now you could work around that and just use the center AF point on the 6D and then later in post just crop to re-compose. I did this and the results were pretty good, you just lost some resolution. For me, I need to be able to rely on the outer AF points for composition right then while I am taking the photo, not later in post, so the 6D was sold. If the 6D had the outer AF points like the lowly T4i has, f/5.6 cross type, then I would still have the 6D today and would need no other camera.
I used my 70D at a local High School night football game the Friday before last, and in between the intermittent rain showers, out of 60 some photo's, all were sharp and in focus; something my copy of the 7D could never do on it's best day. Keep in mind, that I achieved this with just the single AF point, I did not have access to the expanded AF point option like the 7D has. I used the default AI Servo settings straight from the factory for the 70D. This impressed me very much.
The 70D produces sharper images than the 7D does. The 70D has better IQ with low ISO RAW images than the 7D has, think studio stuff or landscape photography. However, the high RAW ISO images like at ISO 6400, are similar between the two cameras. The JPEG images from the 70D are superior to the 7D at all ISO's.
The 70D allows you to micro-adjust your zoom lenses at the wide and telephoto ends where the 7D you can only make one micro-adjustment for the entire zoom range, The 70D, from my experience, has better and more consistent auto-focus than the 7D, not even close to be honest. In all fairness, I have not had time to do any BIF shot's, so the jury is still out in that area. The 70D has a silent shutter option which is very nice, similar to the 6D's. And when not in silent mode, the 70D's shutter is quieter than the 7D's, this includes high continuous shooting as well. The 70D is the first DSLR that makes taking video's fun and makes me want to take video's.
Keep one thing in mind, everything that I just said is only my opinion and nothing more. Just thought I would give you my impressions from my actual experience/take on of these camera's. Good luck with your decision and as you know there is no one perfect camera; the closest to that level of greatness cost around $6800. Way too many confusing and expensive compromises in this hobby.