Essentially, the choice between these two is simple. If you want a shiny snazzy neat smartphone, get an HD2 (or consider an HTC Desire, smaller screen than the HD2 but runs android rather than winmo and a newer Sense UI), if you want a pocketable tablet that also happens to have (very basic) phone functionality, get an N900.
I own an N900 and my girlfriend has an HD2 so I have pretty extensive use with both, and theyre both really neat devices in their own right but are frankly aimed at different markets.

If you do choose the smartphone route though, I would give strong consideration to the Desire rather than the HD2. While the fact that the HD2 runs Winmo isnt really a major issue in my opinion (the platform gets much more stick than it deserves, especially with HTC's Sense UI glossing it up), non-business app availability for Winmo is tiny compared to Android, so you're going to end up with a device that can do a lot more consumer-oriented stuff if you go for a Desire instead of an HD2.