The UI is personal taste. Although the Eris may have a better one, they both work well regardless.
much more compact?
ERIS: 4.45H x 2.19W x 0.51D
DROID: 4.56H x 2.36W x 0.54D
so the Droid is 0.11 inches taller, 0.17 inches wider and 0.03 inches deeper... unless your holding both phones side by side, you can't notice the difference. They are the same size IMO. I also think the Droid feels a lot better since it does have some weight to it and doesn't feel "cheap". I wouldn't say the Eris feels cheap by any means, but the droid feels more expensive and better made IMO.
I personally like the way the Droid looks. I like the raised screen... it's a design element few if any phones have. The Eris looks like every other HTC phone... not bad but "boring" IMO. It's all personal taste though.
Again, the UI is personal taste. I don't really care.. they both work well... and they both fit in your pocket BTW.
In the end, IMO the Droid is better than the Eris, partly due to the keyboard. That being said, I think their value or "Bang for Buck" is the same due to the Eris' great price.