iPhone is the better smartphone. N900 is the better linux pocketable computer.

Get N900 if this is important for you:
- Supporting opensource movement.
- Legal and conceptual freedom to do anything on your device (but you may not necessarily be able to do some of them because of limitations in apps/tools/skills/3rd party support).
- Near complete access to everything in your phone.

Get iPhone if the following is important for you:
- If you want things to 'just work' with less tinkering.
- Great 3rd party support. Sites and apps basically are storming your way, wanting your attention/money (heh).
- Good interoperability with Windows or OSX machine.