I considered this also. The issues everyone have cited are valid.
1) Verizon network. Really my only option in Philadelphia, AT&T is just unacceptable here.
2) Both phones will run the same OS. The only advantage I can see is if Google puts OS features on the Nexus and NOT put it on the Droid. I doubt this would happen, as Google would be "cutting off their own nose to spite their face." If this scenario actually DID happen, yes, it would make sense to want the Nexus, but it won't happen, so its a moot point.
So, with #2 in mind, we come to #3
3) Hardware differences in the phone. I don't think their will be much difference in performance. The Droid is pretty powerful, and I dont' remember anyone saying the processor for the Nexus will be much better than the Droid. I'm not positive about this, however. Maybe some more techies can explain?
So, if we assume the 3 points above is true, then the ONLY reason it boils down to is this : Aesthetics. The Nexus one doesn't have a keyboard, has the iPod-like one button at the bottom, but the google 4 tab buttons above it, like the Droid. It actually LOOKS like an iPhone. Some people like that, some don't. Its a matter of personal preference.
The way I look at it, there is not much of a choice here. If everything was equal, I'd get a Nexus on Verizon. I don't use the physical keyboard hardly ever and I like the look of the Nexus. But in reality they will be running the same OS and perform the same, so the only difference is the "look" of the phone.
-Wil