I think it really comes down to 2 things for most people:
What form factor do you prefer (with or without keyboard)?
How are you intending to use your phone?
The second question is probably more important really. If you are going to use it as more of a multi-media phone for playing games, watching movies/tv shows and snapping pictures, the Epic is probably going to appeal more to you. It has a better camera (even though it's less MP's), it has a better screen, and a better "gaming" processor (especially for graphically intensive games). If those things aren't as important to you as business functionality is to you, then the Evo might be your phone of choice. It's a more classy-looking, business style type of phone that also fits more easily into your suit pants, etc. I'm not saying the Evo is a slouch when it comes to multimedia, but the Epic is clearly a step up in this area.