I own a Motorola DROID, myself. I don't really want to equate the two but, unfortunately, Nokia made it unavoidable to group a phone with what should have been an Internet Tablet. That being said, the N900 is an incredibly bad phone and probably an incredibly good portable computer. I don't like the added cost, bulk, battery drain and don't trust GSM for security so being stuck with the cellular radio was another reason for MY choice to avoid the N900. But if what you wanted to know is where it seems to be better compared to the Motorola Droid in particular... here's my opinion:

I don't own an N900, but from what I've seen, in general (blow-by-blow) it's mostly pretty evenly matched. On the hardware side, it clearly has a MUCH better camera and a much better keyboard than my Motorola Droid. Not that the N900 has a particularly good keyboard--but the Motorola Droid's is ghastly. You can also easily find and use bluetooth keyboards with the N900, which is still unexplainably difficult on any Android system. (I managed to find a bluetooth keyboard and have drivers--but even then it's still awkward.) The N900's CPU is slightly faster, but not by much. Beyond that, I can't think of anything better on the N900's hardware.

On the software side, the operating system is a Debian-based (ie: much more libre) operating system so there's much more flexibility and it feels more like a real computer, whereas the Droid feels like a phone with apps (which, really, is the point of the DROID phones... they're a phone first). To that end, the DROID has many more applications but the same java write-once-run-everywhere ecosystem is also imperfect and doesn't always look or run correctly because of android-specific quirks. By contrast, apps written for the Linux desktop are probably much more easily recompiled and adapted to run without crashing on the N900 (weird formatting and graphics in the translation to small screen aside). Certainly, command line apps and scripts will work with very, very little modification. On the Android side, command line is almost entirely forbidden (although there are many applications to make a command line almost unnecessary, like some excellent SSH apps for example). Tethering is incredibly easy on both (or so I hear, for N900.. I use OpenVPN to tether my Mini9 to my Droid over USB very easily--simultaneously providing Internet connectivity as well as charging my phone.)

On the support front, it seems to me that the phone app and general customer support for the N900 just seems incredibly.. lacking, frustrating or outright awful depending on who you talk to. The Motorola Droid has any number of walk-in Verizon, Walmart, etc. stores you can go to where you purchased it to have it replaced or repaired IMMEDIATELY without shipping involved. Even if it costs money, at least you'll have been taken care of immediately and can walk out with a working phone and never miss a call, a podcast or whatever else you use your device for. Having data everywhere is pretty much a given on both platforms with the DROID seeming to have a slight edge on data speed in the best case scenario for either device.

Anyway... that's all I can think of for now. I rather like my DROID.. but it's just my phone. Not a suitable replacement for my N800 and, despite the cellular phone in the N900, I'm not sure it's right to compare those two.

Hope that's what you wanted.