I am tempted by the HD2 too, so I tried to make a comparison based only on specs sheets and not on winmo/maemo comparison:

N900---------------HD2-------------Comments

32+16Go > 16Go
Stereo speakers > not so good speaker
kickstand > additional accessories(GPS support etc)
5M photo > 5M photo not sure
video recording > video recording
physical keyboard > no
16M colors > 64k colors
FM transmitter > no
tv-out > no
Front camera > no
Price > Price


800x640 = 800x640


3.5' screen < 4.3'
no turn-by-turn < tomtom/navigon/copilot etc... To be solved?
pocketability < pocketability (a lot thinner) subjective
no compass < compass


Cortex A8 600Mhz ? Snapdragon 1Ghz Tests?
Battery ? Battery Tests?
resistive ? capacitive Depends on needs
RAM256Mo->1Go ? RAM 448Mo
micro usb ? mini usb


So in the end that's lot of advantages for the N900, despite the apparences
For me, the lack of FM transmitter and the limitation to 16Go are really minus points for the HD2. And the smaller screen of the N900 is compensated by the same resolution.
Plus the price of HD2 + 16Go Sd card + accessories >> N900
So only considering the hardware, I still pick the N900, even if I'd like the HD2 screen

And I'm 10x more attracted by the novelty of maemo OS than by the HTC touch flo that I've already use for too long (even if the new version seems cool)