I concur with PJL.
The XS is simple the least expensive DSLR in the line up. It's got the least amount of features etc. Compared to the others, the only attractive bit is the price.
The T3i, is the latest, greatest generation of the Rebel family. I don't know the specs, but I'd guess that's it's much more well apointed than the XS, and probably a good bit more expensive.
The 60D is (or is supposed to be) a whole level above the Rebels. It's bigger and the economics will be quite a bit different. In other words, the layout of the controls is different than the rebels, and there is more control at your finger tips, where as with the Rebels, you would need to go into the menus for the more advanced features. That was certainly the case when the 20D, 30D, 40D & 50D were current. The 60D is has diverged from that line and it's actually much closer to the Rebels. It's still a better camera than the T3i...it's just that the 60D isn't 'as much' better than the 50D was better than the T2i etc.
The 7D is the next step up...and I think Canon made the 60D the way it is, because now it fits perfectly in the middle of the T3i and the 7D.
When it comes to choosing which one to get. I think you need to go over the features. If you need something that the higher level camera has, then that's what you should get. But another important factor is the size and the ergonomics. As such, I'd recommend going into a camera store and holding the different models. You may find that one is too small or too big etc....and that may be the deciding factor for you.