Isn't the HD2 running a dead-end OS? WinMo6.5 is NOT compatible with Windows Phone 7, and WP7 upgrades are highly unlikely for old HTC Phones (where "old" for HTC means anything that has shipped.)

Otherwise, I would pick the n900 over the HTC for build quality (even with the detaching USB plug) and I would pick Maemo5 over WinMo 6.5 for dead-end OS.

If you're considering a different phone, go with something other then WinMo 6.5.