The Evo gets the android updates quicker because it's an HTC phone. Samsung has been total shit at getting out the 2.2 Froyo update to the Epic (and all the other Galaxy S phones in America). Evo screen is 0.3 inches bigger than Epic, but Epic screen is much better looking.
If you go to the store to test them, try out the Swype keyboard. For sending texts, it's really great if you get used to it. Basically just trace out a line over the letters spelling the word you want, picks the right word 95 percent of the time. If you plan on using the phone to send out long emails though, I'd recommend having a real keyboard.
When I was upgrading my phone, I was in the same boat as you. I ended up just going to the store and within 20 minutes of trying both phones it was an easy decision for me.