Its not really a "which one is better" its just that the iphone and n900 do similar things in very different ways. With the N900 I am free to communicate how I please. Tether, skype over wifi or 3G, text, IM, etc. Its all right there and I can easily download even more connectivity options (multiple IM protocols, etc.) I also think the n900 notification (the way messages "fly in" on screen without halting whatever you're currently doing. That feature is the best of any phone.
There are some nice high-end admin capabilities as well. N900's multitasking is very good, though it can get choppy at times.
Also, I love that there is a ton of storage built in, 32 GB is not an option for an iphone. Also, you can't replace the battery. And you have to use iTunes when you connect an Iphone 4 to your computer... i love the mass storage mode.

The iPhone does win in presentation and overall "responsiveness" And even though its camera is similar to the n900, I believe the software produces better pictures. And the sheer variety of apps is impressive. But that comes at a cost in not being able to use the phone in certain ways without either Apple or ATT's blessing.