Pros [of the Droid on Verizon]:
1. Greater higher speed data network access than (most likely) anyone else in the US. Verizon has great coverage. This is a huge pro for some folks but IMO overrated for many.
2. Java. It'll be here soon for the N900 but not today.

Cons:
1. It's a Motorola. Once they built nice mobiles but IMO not anymore. When Nokia wants to build first class hardware they can and do it better then anyone else. The fit and finish on my trusty E61i is impressive. I have yet to find a Moto device that compares.
2. Locking users and most apps out of the GPS chipset ( http://en.wikipedia.org/wiki/Verizon...ertain_devices )
3. Verizon customer service, while better than AT&T is still a far cry from TMO. I've been with TMO since the VS days and their customer service has always been excellent.
4. The Droid is mostly new while many parts of the N900 have been in use since the 770. New designs are never perfect (like the WSOD on the 770). The N900 should have less defects due to it's maturity.
5. Android is still pretty locked down compared to the N900. The idea of buying hardware and having to hack though useless layers of security to run my own stuff is odious. My money, my hardware. While Android is better then most it's still not as free as the N900.
6. No control key. How are you suppose to use emacs? =)

Either way:
1. Cost. While the unsubsidized price will likely be less than the N900 the subsidized price could be more considering Verizon's rates [compared to TMO].

In the end I believe the two devices mostly address different audiences and are not strongly competing against each other. It looks like from the ad I saw they're directly going after the iPhone which IMO is bound to fail. No one beats Apple at a game they created.