Shun the N900. It's an EOL (end of life) item and despite it being incredibly able; it does not have a lot of consumer level items - read: support, popular and modern games, gaming frameworks like OpenFeint for instance - and ultimately you're tied to this (albeit awesome) community, you'll not have items that will get upgrades like Skype for instance.

Shun the HTC Desire. The build quality, while decent, isn't great. Expand your options if you could. Good luck with your endeavors.