Before upgrading, try rooting your hero and see if you can hold out until Q1 of next year, good stuff is on the horizon.
Otherwise it's all personal.
Evo- BEST for rooting, great screen size, solid hardware, well built, sense (yay), kick stand, htc provides better support, bigger on screen key board, and non hiding capacitive buttons.
Epic- awesome physical qwerty, that super amoled is sexy, touchwiz (puke), kind of plastic feeling, better gpu for gaming, samsung support sucks, the capacitive buttons are wonky and it's on 2.1 for now. But don't get me wrong it's a great device, especiakly with root.
Honestly you can't go wrong, both are top if the line devices, and both have FAST 1ghz processors.