Good questions. Here are my thoughts:

N900 keyboard is better than that of the Milestone
N900 has front facing camera and irda
N900 makes it extremely easy to install .deb files

I think the hardware on the N900 is superior to the Milestone. I would imagine that it will be possible to easily dual-boot Maemo(MeeGo) and Android in the future. That way you get the best of both worlds.