I’ve only seen a Droid in passing, but have used the G1 and N1 extensively. Here’s my take on why the N1 is great:
The Screen: It totally rocks. Great contrast, beautiful colors, nice and crisp. The only drawback is that you need to crank up the brightness when viewing outdoors, but I find it rarely ever do this, and it does have an auto brightness control. The touchscreen accuracy of the N1 is supposed to be significantly better than the Droid as well.
The form factor: Round and curvy, it’s great.
Performance: I’ve heard that the N1 is much faster than the Droid at web browsing, but I haven’t seen a side-by-side.
Lack of Contract: Going contract-less on the N1 is great. I hate phone contracts, and the N1 lets me do this easily. The price of the phone is steep, but if you look at the long-term cost benefits, I think it’s much better, especially if you want to switch phones every year.
3G: I’m happy with T-Mobile’s 3G coverage in my home area, but when I travel, it seems pretty spotty. Verizon on the Droid is going to be better for this.
Camera: I don’t think the Camera on the N1 is really any better than that on the Droid. Neither suffice to replace a “real” digital camera, and both are fine for posting to twitter, facebook.