I concur with others that the best thing to do is to go into a VZW store and try out both phones. In all of the stores that I have been in, they are side-by-side, making comparisons easy.
This is what I did - I actually went in twice before I made my final decision. What it came down to, for me, was this:
1. The Droid felt clunkier in my hand. It is heavier (due to the keyboard), it felt all angles rather than nice curves, like the Eris.
2. The Droid's hardware keyboard, in my opinion, is very mediocre. It reminds me very much of my old Psion Revo, and I grew to hate that keyboard. It was too difficult to discern whether or not a keypress was actually registered - there was no point where you knew for sure that you'd hit the right point in the keypress - and the Droid felt exactly the same to me.
3. The virtual keyboard on the Eris seemed more accurate to my touch than the on-screen virtual keyboard on the Droid. In my tests, I was making far more mistakes on the Droid.
4. I preferred the Sense UI shell that is on the Eris to the Droid's vanilla 2.0.1 Android shell.
5. That said, I preferred Droid's screen and Android 2.0.1 to the Eris display and Android 1.5 on the Eris.
6. The Eris is $100 less expensive.
Based on that, I decided on the Eris. You may disagree with some of these based on your own use, or decide that there are better reasons for the Droid over the Eris. (The Droid has more RAM installed, and comes with double the storage on the SD Card - some other things that may persuade you toward the Droid over the Eris.)
So, again, go in and try them out for yourself. Hopefully you will have an easier time deciding than I did (I went in with my family so that all four of us could get upgraded handsets, and I was taking a bit longer to decide than my wife appreciated...)