Things to consider. First of all, the bigger screen of the Evo does not put it at an disadvantage compared to the smaller screen of the Epic when it comes to overall sluggishness since both processors have the same amount of pixels to throw around since both phones run a 800 x 480.
Next on the list is the OS, Froyo runs better than Eclair and since, as of now, the Evo has Froyo and the Epic is still playing around with 2.1, the comparison again is a bit offset, this time in the evos favor.
But, hardware for hardware, the Galaxy S phones have the upper handle, the Hummingbird is just way faster than that Adreno POS we have in the Evo, so for games, and live wallpapers, the Epic wins, even with the older OS. Once the Epic has 2.2, the winner in terms of speed and smoothness will be crystal clear.
Also to consider are the two custom UI's, HTC Sense vs whatever Samsung calls it's iPhone imitation looking UI.
The Epic definitely has longer legs in the hardware department, it only falls short with the lower MP camera and smaller screen size, but makes up for it with better screen quality.
Tough call.