Pinch Zoom - Available on Droid via 3rd party apps (Dolphin Browser, alternate "Gallery" apps, etc.)
Track Ball - Tried to use it, but it was not great. I didn't use it once except for testing purposes when I first got the phone.
HTC SenseUI/Widgets/Apps - This is where the Eris shines, but it wasn't enough to keep me from exchanging for Droid...
OP. It's really up to you and which device you prefer. There are upsides to both phones.
Eris = Smaller footprint, SenseUI (VERY NICE)..I think the smaller footprint plus more of an "iphone" feel attracts many users...
Droid = Bigger Screen, Better Rez, Hard Keyboard (I don't use this much), Flash for the camera, faster processor (very noticeable to me), 2.0 (for now, that is still a factor, but won't be once Eris gets 2.1)