If you use only the center point, the T2i/T3i's center point is cross type with glass f/2.8 and faster, the 60D's center point is dual cross type (it's crossed with both a "T" and an "X", making it sensitive to horizontal, vertical, and diagonal lines) with glass f/2.8 and faster. The 60D's center point will be more sensitive and catch focus better in low light.
That said, if you're not having issues with your XS catching focus in low light, T2i/T3i may be fine.
I want to get the best camera I can afford that will last and not become out-dated quickly.
I wouldn't worry about that, the T2i, T3i, 60D, and 7D all share the same sensor, and they'll all equally be outclassed by the next generation of cameras
. More importantly, they'll continue taking great pictures well past when the new models come out.
The only differences between the T2i and T3i is that the T3i has a swivel screen and wireless flash control. If neither of those would help what you shoot, look at the T2i first.
The differences between the T2i/T3i and 60D are as follows:
- bigger body
- some weather sealing
- second control dial on the back
- top LCD panel
- faster burst FPS, deeper buffer
- longer battery life
- bigger pentaprism viewfinder
- better 9-point AF system (all nine points are cross type, center is dual cross type)
If that sounds like it's worth the extra few hundred dollars over the Rebels, the 60D may be the way to go.
The differences between the 60D and 7D are:
- even bigger metal body
- even more weather sealing
- joystick instead of directional pad
- 8FPS burst mode
- 19-point AF system
- AF microadjust
For those that need these things, it is totally worth the extra money for the 7D over the 60D. I don't think that you sound like you need those things, though.