Some great posts here. This thread is a good example of the problem with choice...
Traditionally, technology always gets better and the only reason for a company to release something new (a 2nd choice) is because its better. So adding a new choice becomes a straightforward newer = better or money = more. Or better yet, the old choice is obsolete and you are left with one new choice. Sure the 1st investment is now worthless, but at least the choice of what to get next is easy! But that didn't happen here. FF sensors were (and remain) more expensive, to the point that many early adopters (myself included) had to 'downgrade' to smaller sensors when migrating from film. And we seem to be stuck with 2 complete and somewhat convergent platforms - indefinitely.
Theoretically, the only reason a smaller well built sensor, should be inherently inferior to a larger well built sensor, is if something happens to light rays, where a sensor is so small, not as many photons are available to reach it (say photons per pixel per second). So say canon made two complete and independent lines, with the mini line being half sized (top to bottom), but having 100% of the same bodies and lens models at the exact same prices. You could have everything from a mini wide tilt shift, all the way up to a mini equivalent to the 1200mm super tele. With same quality glass and sensors, cameras behave the same and photos look the same. Which would you then choose?
Reality is somewhat like this, EF-S are smaller for similar performance, but thats equivalent to longer non L EF lenses. If you want L glass, you still pay L prices and still carry around L grams. You just don't get the 'full advantage' of the glass you buy. And its easy to say that IQ of a 1 year old 5DIII bests and 5 or 10 year old crop (that even knew was a quarter the price), but that isn't really an apples to apples comparison. The closest examples of two cameras from each of these formats is the 6D and 7D. Assuming equal bodies (the 7D has somewhat better controls), the extra cost of a 6D goes largely into the bigger sensor and wireless systems. So how do they compare?
Well, I just got the 6D not 2 weeks ago and took these photos this very week. Same table, same lens (16-35 mki), nearly the same lighting
A shot of a 7D, taken by a 6D:
and a shot of a 6D, taken by a 7D (which I just sold, using the above pic):
IMG NOTICE: [NOT AN IMAGE URL, NOT RENDERED INLINE]
Strictly speaking, I don't know that one bests the other under these conditions (which were not chosen to find sensor differences). The 6D is a really good camera. The 7D is a really good camera. If you have a stable of EF-S lenses you love, the 7D is a great way to go. If those were just the best you could get at the time (whatever the reason) and you feel you've outgrown them, why not step up. As long as Canon continues supporting both lines with robust options, go what what you love/need.
Do I love the 6D, yes. Did I love the 7D? Not really, but not because it wasn't an excellent performer. Bottom line: with the advent of quality affordable sensors on both platforms, the choice comes down to which lenses you want and what size sensor you prefer being behind them.