In general th Epic is a much better device. I owned the Evo from launch date until three weeks ago, when I got an epic for the GPU prowess.
Come to find out, the Epic performs better, has a FAR FAAAAARRR better camera, especially the video camera, has a much MUCH better screen, the touch screen is much more responsive, better sound quality and more. I could write a book about how much happier I am with the epic. I finally don't long for an iphone.
However, there are a few bugs. One, I notice that I have slower 3G speeds on the epic when compared to the evo. Also, some key presses are missed by the keyboard. I used to have issues with gps, but the last update fixed that. Other that this, I hav not had any big issues that I can think of.
if you don't care for an extra .3" of LCD screen, go Epic. Just about the ONLY thing evo has better is Sense. Which I do miss.