I'm assuming you can wait until the Nexus One hits Verizon and you can take the carrier out of the equation, but Verizon's network is a huge pro for the Droid right now. I researched both phones because I considered trading my Droid for a Nexus One when it arrived on Verizon (mainly because I like the form factor better), but honestly, the differences between the two are negligible. Here is what I've come up with:
Nexus One:
+ Faster processor and more ram compared to Droid.
+ Full-featured Android 2.1 OS (nobody knows if the Droid will get this).
+/- AMOLED screen (but it doesn't perform well under sunlight, and has some contrast/saturation issues).
+/- Thinner "candy bar" form factor w/ trackball (but no hardware keyboard).
- Soft back/menu/home/search buttons are unresponsive on the lower half.
- Problems with multi-touch implementation (as seen by newest videos).
Droid:
+ Higher resolution screen compared to the Nexus One (854x480, which is true widscreen resolution, vs. 800x480).
+ Comes with 16GB SD card compared to the Nexus One's 4GB card.
+/- Thicker, heavier form factor, but has a hardware keyboard.
- Slower processor and less ram compared to the Nexus One.
I decided it wasn't worth it to "upgrade" to a Nexus One from a Droid because their pros/cons are pretty much a wash. I think the only real valid reasons to go with the Nexus One at this point are if you prefer it's form factor and/or you really must have the 1Ghz Snapdragon processor and 512MB of RAM. The differences between the two phones are negligible and the Droid actually beats the Nexus One in certain respects.