I am going to do exactly what you say, but havn't yet. Probably next week. My brother got a Droid, so I've been able to mess with it...
1) The screen is bigger (and amazing with the higher resolution). That is not to say the Eris screen isn't great...It is! It's just smaller, with lower res, and I am a "bigger is better" type...lol
2) The Processor is faster. On my brothers phone things are noticeably faster. By quite a bit actually. I noticed this first when we where downloading an app at approx. the same time (not done scientifically, admitted). His app was done downloading AND installed before mine was done downloading and I started first! We where both using WIFI at my house (Fios 25/25 connection, so no bottleneck there)...
The thing I will miss the most is Sense UI...I really like it, but Android is so customizable with apps, that I should be able to get close...