Went from a 40D to a 50D when the shutter failed on the 40. Same camera basically, slightly more resolving power, same high ISO performance overall, but with microfocus adjustment, which I consider a must on a DSLR.
Burst speed is the big draw of the 40/50D compared to the consumer bodies. If you don't need fast bursting and MF adjustment then the 100D will do pretty much everything you need in a much smaller package.
I'd avoid the 60D personally, durability took a step backwards and Canon removed several features, including MF adjustment.
70D is your best friend, I'd say it's the sweet spot in terms of performance and value.