The Epic is a better choice and here is why.
1) If its not your main phone you'll have easier time rooting and loading custom roms on an Epic..its fairly simple and its nearly unbrickable so you don't be afraid to mess up.
2) It has better hardware from AMOLED screen to one of the best Audio DACs.
3) It has a better camera.
4) Better processor and 4G idle
As far as updates go, I don't think it matters much because I don't think either Epic nor Evo will get any more major updates. There is a rumor that Samsung is going to update the Galaxy S to ICS but even if they do the question is whether sprint will bother.
The Evo does have a larger development community, that said its not like you'll run out of things to flash on the Epic
lol