I'd also go with the iPhone, there aren't really any apps for Windows Phone yet. Had a buddy who "upgraded" to a Windows Phone 7 (7.5) like a year ago. He went back to using his old iPhone 3GS after a month, just to many things he couldn't do with it. I like to rub it in and asked him if he was going get a Windows Phone 8 last week, lol, it was a pretty stern NO! hahaha. I'm sure Windows Phone 8 will be better but I sure wouldn't want to go through the early adopter pains with a lack of apps.
I actually recently just for the heck of it went through all my major apps just see what there and what was missing (I actually kind of like Windows Phone UI). I found that Windows Phone didn't have apps for most of what I use. What was amazing was Black Berry actually had apps for almost everything I do.
I'd suggest doing the same, make a list of the Apps you want/need and compare off that. Both OSes are good and the hardware in both is good (performance wise). So, it comes down to apps and looks/dimensions/weight.
However, if it was me, I know I'd be happy with the iPhone, maybe I'd be happy with Windows Phone??? So, I'd go with the safer bet.