In reply to Durbs: to be fair the 60D is a bit smaller & lighter, but yeah Durbs basically the same size. But the 60D is a plastic composite body - not ideal for outdoor photography - compared to the 50D's rugged alloy body (it's like a mini 5D). The 60D marked a shift from Canon's XXD range. It even has a swivel screen (something else to go wrong?). The 60D's smaller form is more like the XXXD Ti rebel thingy range really.
In a nutshell the main advantage the 60D has is an improved sensor and iso performance, and better LCD. I think the former two points are deal breakers for most people. There are many other more minor improvements. One must scan the specs and decide if this is needed and worth the extra cash i suppose.
The 50 & 60D have the same autofocus and same shutter and mirror. Advantages of the 50D, in addition to alloy body, is that is has a faster fps and also a manual lens adjustment built in (60D for some reason does not). Another important consideration is that the 60D doesn't take CF cards, which is a serious pain and financial blow if you have a nice collection of xGb CF cards (I know I wouldn't buy a second hand CF card).
Both are great cameras and I'm sure you wouldn't be dissapointed with either... Unless you drop your 60D when your fancy LCD screen is flipped out
Apologies for the bias review!