I've owned both phones. I'm sure you'd be happy with either one of them. The biggest difference, and the one that you should probably care the most about, is the keyboard. I liked the Droid keyboard and it was a little sad to go to the N1 with virtual KB only. If you need a keyboard, get the Droid. If not, get the N1.
Yes, you can root and OC the Droid, but the N1 is 1GHz stock. My friend has a rooted Droid at 1.2GHz and it is faster than the stock OMAP clock for the Droid, but I don't feel like it is faster than the N1. I could tell that the Droid was slower, not horribly bad, but noticeable. Clock speed isn't everything, and I think that the CPU in the Droid isn't doing the work per clock cycle at those speeds that the N1 is.
We don't know if VZW will want more control over the N1 than T-Mobile has. That could be an interesting point. The openness of the N1 is one of the big selling points. If they're going to try locking down the N1, that could affect the speed of updates. Might just want to wait for one of the high end HTC phones coming to VZW with Sense UI.