Sprint put out a short video (below) to break down some of the features the Epic has that the EVO doesn't have like a Super AMOLED display, slide-out hardware keyboard, and DLNA. The Epic push is obvious and to be expected since it's new, but its specs aren't unequivocally better. The EVO has a bigger screen, higher resolution front and back cameras, an HDMI port (this is mentioned in the video but not really in a positive way), kickstand, and of course is already running Android 2.2. On the other hand, the Epic will presumably be updated to Froyo soon and in addition to what was mentioned earlier, it's got a more powerful CPU/GPU (Hummingbird/PowerVR SGX540 vs Snapdragon/Adreno 200) and comes with a higher capacity microSD card.
It's all a matter of preference, really, and I don't think one is necessarily better than the other. One is probably better for you, though, so which is it? Will you be selling your EVO to buy the Epic at full price? Signing up with Sprint for the first time to get the Epic instead of the EVO you were thinking about getting? Buying both? I'm thinking about getting the Epic because I'm returning my Samsung Captivate and still want a Galaxy S phone, but I know I don't represent the normal single-phone-using consumer (I'm currently using five).
Thanks, Lag2oon!