I agree with you that the HD2 might be a better internet tablet from a hardware point of view.
However these are the things I like better in the N900 :
- 32GB flash memory included + microsdhc slot
- probably better internet browser
- probably better camera
- hardware keyboard
- FM transmitter
- might be cheaper

Maybe I'll consider the HD2 if USB host is working well on it, for example to read big usb memory keys ...