Well, i looked up the HD2 and it's kinda hard to give a definitive answer. The hardware is pretty much the same, with 2 notable differences:
*bigger cpu frequencies and more ram on hd2: note that this isn't necessarily a big advantage; the cpu's are different architectures and wikipedia says that arm cpu's are more efficient, even with lower speeds; and the extra ram will probably just be compensating for the lame windows mobile
* screen: bigger (which obviously also means bigger phone) and capacitive; it seems however that windows mobile doesn't support multitouch, which htc says their phone has
Otherwise, the hardware on N900 and HD2 is pretty much the same. So is the price. The big difference is in the software: Maemo 5 vs WinMo 6.5 . I haven't used any, so i can't give a first-hand impression. What it is clear to me is:
- as far as i remember, there is no real multitasking in winmo; apps stay where they were when you switched, as opposed to going about their business like in maemo
- maemo ~ linux, which means that it can share things with linux desktop platforms, like ubuntu; while this may not be that great for a non-techie, it can provide both software and help from the desktop world
All in all, it's a choice: do you want freedom (maemo) instead of having your hand held by software vendors?