I've had two 7D's and I've now got a 70D. The 7D's were really used as a backup to my 5D3 but the 70D is much more a companion to my 5D3. The 70D gets far more use than the 7D ever did cause I couldn't get any really good clean shots from my 7D's above ISO1600. With the 70D I can go up to ISO6400 and still come out with cleaner photos. The 7D AF system is slightly better than the 70D, in that it has a smaller spot focus point than the 70D, but other than that it's about the same. The 7D had more setup options than the 70D does but that can also be its downfall, the 70D is much more forgiving than the 7D. If you under/over expose slightly it produces cleaner photos than the 7D did, at least the two 7D's I had anyway. I also had some focus and metering issues with the first 7D, it would lose focus or change exposure in the middle of a burst so I'd have some good and some bad from the same settings at the same time.
On the whole, I'm much happier with the 70D than I was with either of the 7D's, I can rely on it to perform well every time. I've used both EF and EF-s lenses and had no focus issues that have been talked about on forums. The AF speed of the 70D and 100 f2.8L Macro is fast enough that I can get over 90% of my shots using AF instead of manual focus. The screen is excellent, the ability to position it just where you need it and to be able to use the touchscreen features is a major bonus (specially for macro shots and above crowd shots) and the Liveview features and video are probably the best Canon have produced so far. The most important thing I found is that I've still got the 70D, and I love using it, whereas I've not got either of the 7D's anymore. That probably tells you more about my opinion of which is the best between them.
I've not used a 6D yet but from the reviews and the examples I've seen on various forums and websites it would be worth considering if you don't need the 1.6x crop.