I've never used the moment, though I've heard it has a harder time than the Hero since it only got basic hacks on 1.5 to get CDMA working. I love Sense UI. I guess that is the biggest difference. If you like Sense UI and no keyboard (therefore a smaller build), the Hero is great. If you want a keyboard and stock Android 1.5 with its own set of issues, get a Moment. Also, I don't know about the developer community for the Moment, but XDA has a great variety of developers and hacks for the Hero. Basically, its the most hackable phone after the G1. Oh, the other thing is if you like Samsung phones or the slightly higher grade HTC phones. Samsung does make good phones, but they always feel a bit more cheap to me.
Edit: I almost forgot, the Evo will destroy pretty much all phones on all carriers, so definitely go for one as soon as you are financially capable. I'm going to. Love my Hero, but Evo makes it look like a caveman!