i guess it just depends on what you want.
how old are you?
the iphone insists upon itself to the point where you can say enough. it is a fun phone, but there are alot better phones out there.
if you are younger, and not opposed to disposing an extra 30 bucks a month just to have the iphone, plus your minutes and texts (a decent bill from att will never be less than $100 with an iphone) then go for the iphone.
windows mobile is great, and the new version 6.5 is more stable and has the marketplace. they dont have nearly as many apps as itunes, but they are barely getting started with their apps, so there should be alot more to come. not to mention that you get a variety of phones to choose from, including the soon to be htc touch hd2 (its sick man!).
i currently have android. kicks the iphones derier, and it has alot of apps. plus its cheaper.