The Nokia 5800 is probably one of Nokia's better phone's that they have introduced for a while but just be aware that Nokia's usually have allot of bugs since they started upping their game in recent years on the phone market. I used to own a Nokia "N" series phone, 2 of them to be precise and with both i always had problems, so did my friend who owned a different Nokia N series, so my faith with Nokia fell quite a bit.
Apple are more frequent with their updates and in fairness the phone works like a dream, its a polished piece of kit. Apple do also have allot of after market support for their products from many manufacturers and there are so many add on's you can buy for the phone its crazy, such as an ipod dock, there's loads of them, thats one example from many.
If i was you, i'd head down to a Apple shop if there is one near you and have a play with the iphone, also see if you can play with the Nokia 5800 at a Carphone Warehouse store or somewhere similar and see what you think of that too.
Everyone here on the What Hi-Fi forums are going to tell you to grab the iphone, it won the readers product of the year.
Regards> M