Yes, you are comparing a crop sensor camera with a full frame camera. They are different animals, each with their own strengths and weaknesses.
Taking the crop sensor models first....
The 7D is a crop sensor camera (same format as your XTi), sort of a pro-oriented with a fairly sophisticated focus system. It generally requires more experienced user input all the way from camera setup before the shoot through image optimization and finishing in post-production.
You didn't ask about it, but 70D is a four-year-newer model with improved AF in Live View and video modes, slightly downgraded viewfinder AF (three modes compared to five in the 7D), and a mildly improved sensor/image processor. It has some support for inexperienced users... at least more than 7D.
The now discontinued 60D uses essentially the same sensor as the 7D, but has an easier user interface than the 7D (more support for inexperienced users) and less complex AF system than either 7D or 70D. The AF system on 60D looks the same as your XTi, but actually is improved in that all nine AF points are the better "cross type" (your XTi has only one cross type, the center AF point, while the other 8 are simpler single axis).
7D, 70D and 60D all can use any lens that Canon makes, both EF and EF-S, same as your XTi.
Now consider the full frame model....
The 6D is full frame (like the 5D series, for example). As such, it can only use FF compatible lenses: EF (not EF-S). Full frame compatible lenses, on average, tend to be larger, heavier and more expensive. The 6D is a somewhat simplified, more entry-level FF camera (compared to 5D Mark III), with some support for less experienced users and a less sophisticated AF system. In fact, it's AF system looks similar to your XTi's, except with 11 points (one cross type, the other 10 single axis.... similar to your camera, although all of them are more sensitive than the points in your XTi).
So, even though these are all DSLRs and share a lot featuers, you are sort of asking to compare apples and oranges.
6D is a very good low light camera. Like most FF models, it tends to have less noise than crop sensor cameras at similarly high ISOs. Also, using center AF point only, it's able to focus in approx. 2EV lower light than 7D or 70D. It is okay for action shooting, but not ideal. Would be nice for landscape, architecture, portraiture, and perhaps macro and "street" photography.
7D is more of a sports/action camera. It's a fairly heavy, sturdy camera. It actually weighs a few grams more than the 5D Mark II full frame camera, and is about 20% heavier than the 6D.
In a sense, the 6D is a full frame version of the 60D.
The 70D is a step up from the 60D.
In some respects 7D is a step up from 70D (though in a few other ways the 70D is a step up from the older 7D).
For example, 60D, 70D and 6D all use an 8-way switch to select AF points. In comparison, the 7D (5DII and III, 1DX, etc.) use a "joy stick".
60D, 70D and 6D all use SD memory cards.
7D (and 5DII) use Compact Flash (5DIII has dual slots and uses both types of memory).
70D, 7D, 5DIII and 1DX all have "active matrix, transmissive LCD" focus screens. These reconfigure themselves with different AF modes and are not interchangeable.
60D and 6D use standard AF screens, user interchangeable.
If you are an experienced, sports/action shooter... knowledgeable about setting a camera up and into post-processing your images, go for the 7D. 70D would be a slight step down from that in some respects, but a step up in others.
If you are a less experienced shooter, accustomed to using the highly automated modes and happy with your images straight from the camera, or if you shoot in low light a lot, and mostly stationary subjects, the 6D might be a better and easier to use choice....
Unless you have a bunch of EF-S/crop only lenses and want to keep using them (or aren't prepared to spend additional money to replace them with FF compatible lenses), in which case you might want to instead look at a 70D or 60D.
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Alan Myers
San Jose, Calif., USA
"Walk softly and carry a big lens."
GEAR: 5DII, 7D(x2), 50D(x3), some other cameras, various lenses & accessories
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