I've been trying to make this decision myself recently. I'm not a fan of WinMo, but Sense may hide it well enough. The pros I'm seeing in the HD2 (for me) are the screen, overall thinness, and GPS navigation. For the N900 I much prefer Maemo, the camera is better, and the browser is superior.

I find myself going back and forth between the two everyday, but need to get a new phone. For those of you with the HD2, did you ever try the skyfire browser and can you compare it to he IE, Opera, and N900 browsers?