They're fairly similar. Both are fast, but the Epic is faster. The Epic has Swype preloaded so I never really use the hardware keyboard. But it's still useful for some things, like gaming. Emulators are 100x better on the Epic because of the keyboard.
Battery life is fairly poor on both. I think the Epic's screen looks better; the Evo's still looks good but it is more washed out.
Honestly they are both great phones. If I had to choose, I would pick the Epic, but the Evo is still great. Also, Sense is much better than TouchWiz, and HTC has more support for their phones. We're still waiting for Samsung to release 2.2 for the Epic; it's been leaked but it's not officially available yet. Meanwhile, the Evo got 2.2 months ago.