(The last thing you want to be doing while doing your own dev work is wondering if the camera/accelerometer/whatever API calls are failing because of a sketchy driver or whatever.)
(Even if you're not targeting right now 2.2, you still want to have the ability to test on it.)
Note that despite the confusingly similar names, the HTC Droid Incredible is completely a different phone than the Motorola Droid(s). Google themselves recommend to get the HTC Incredible if you cannot get the Nexus One.
However, bear in mind I haven't actually gotten the phone yet. Better ask me again after I've gotten the phone whether it was a good idea or not. At one time I also thought the Freerunner would be a good idea, but as a phone it was so flawed as to be unusable.
Downside, however, is that any particular advantages / disadvantages won't really reflect the broader Android install base, since there are so few Nexus Ones. But there are relatively few serious problems that come with fragmentation when you are considering the 2010 crop of Android phones -- between all the big names, everything should work pretty well.
Getting a Droid (the original Motorola one) can give you a taste of what many, many people have and are using, same can be said if you got an Evo, DroidX, etc... And it can be a worthwhile experience to see how performance might differ when burdened by the skins (MotoBlur, Sense) that are on the phones as well.
So if you do not want to be stuck with whatever firmware version Motorola offers, get Nexus One.