There have been a bunch of these threads and after reading most of them I will tell you this.
Set aside an hour to two hours and go to the Sprint store, try both phones. Play with the apps, heck even install things from the market if you want.
Take pictures and compare the quality. Experience the differences between the two launchers, see how the epic 4g fits in your hand when using the slide out keyboard for typing and try the htc keyboard out on the Evo. If you're a "communicator" then really get a feeling for how each of them feel in your had.
They're both fast, bad a$$ Android 4g phones.
One caveat that I would mention though, is if you plan to root and hack your phone then check out the development sections both here and at XDA. IMO there's a strong htc custom development community, don't get me wrong the epic has it also but to me, it's not as popular.
A lot of the phone you pick will be how it feels to you, each of these phones are plenty capable. that being said you'll have 30 days to exchange it as well if you choose one and don't like it as much.
You'll end up with one of the best phones either way!