They're both great Android phones, but it's really going to come down to four things-
Price (The Eris is only $100, Droid is twice that)
Real Keyboard (Droid's got one, Eris don't)
Size (As you said, the Eris is smaller. The Droid is just a tad clunkier, but such is the cost of the keyboard)
Free Turn-By-Turn Navigation (Exclusive to the Droid so far)
The functionality, apps, network, and other factors are pretty much the same.
But if all that seems pointless to you, just hold them both in your hands, and see which one feels better.