The 7D has a much more sophisticated autofocus system and comes in a "professional body" - which is more rugged, weather sealed, rated to a higher shutter count, etc. I don't doubt it's more durable, but I spent a year with a 450D (early predecessor to the T3i) smashed into a pack with my rockclimbing gear as I backpacked through humid South East Asia, and that camera is still working fine to this day. It's got over 100,000 actuations on it, which is over twice it's labeled life. My point is simply: don't read too much into it, people grossly overstate specs on cameras.
If your intent is to spend a lot of time in the elements doing photography, then the 7D is absolutely the choice. But a lot of casual photographers convince themselves they need these rugged bodies and the camera spends its life in a comfy camera bag only to come out for birthday parties.
Other specs worth noting: THe 7D has a much faster burst rate (8 fps?), and a bigger and much better view finder. The T3i on the other hand has the flipout LCD screen, and is smaller - something some people actually like. The 7D uses compact flash, the t3i uses SD cards. The 7D absolutely has more features and settings, but a camera with a bunch of settings you don't use isn't any better than a camera without those settings. It all depends on how you intend to use it.
