Each has its own pros and cons. Buy both of them if you can afford it. Droid4 is a bit more future-proof, I guess, due to its CPU/RAM.

Main Droid 4 problem, - currently no acceleration, which slows down hildon. However this is being worked on.

N900 is let down by the ram/cpu combo mainly, and then getting audio from voice calls is going to be another PITA aiui, but otherwise support for it is really good.