Internet is a bit better than iphone on the N900, camera is a fair bit better IMO.
I use a mac (itunes) and the N900. But then just have all my music on the N900.
I'd not carry either riding tho incase it got wet/broken. Got a cheap nokia for that.
If the openness and getting an xterm don't appeal/mean anything to you, get the iPhone. £8 difference per month over 2 years would be nearly £200. You can go cheaper still tho, http://www.hotukdeals.com/item/618367/nokia-n900-600-free-mins-500-texts- etc.
And I'd say have a play with an N900 if it appeals, or make sure you can return it if you don't get on with it. Likewise the iPhone (shock horror not everyone likes them!)
Also I'd not recommend the N900 to anyone not prepared to reflash their device for the lastest OS upgrades. If that means nothing to you, I'd probably steer away…