It's pretty much a meaningless question. They both do different things, and where their features intersect one is better in some ways and worse in others. "Apples" and oranges, y'know.

Asking if iPhone is "better" has so many variables, you're guaranteed a subjective response. You'll do better to ask about specific features that are important to you.

I suggest that you look at some of the many youtube videos demonstrating N900 features. Those will give you a good idea of the UI, and the device's size and responsiveness.