I had both. Now only have N9.
N900 browser supported flash, and also reported itself as a full browser i believe while N9 has no flash and defaults to mobile website version. Otherwise N9 is plainly faster. Also N900 stylus allows for more precise tapping, as well as selecting text to copy.

Otherwise both devices have unreliable OS. I lost all my SMS on both devices randomly for no reason. Should have bought a dumb Symbian or lame but reliable iPhone. ****en hate my choice to give N9 a try after lost all sms today.