I'm anti apple, and a android fanboy...I admit.
I've been in the tech industry for about 15 years, so I have my reasons that I wont go into.
If you're a tech geek, that likes TOTAL control over your device, AND have a respect for opensource material......There is nothing other than android.....and as far as the UI....I honestly have no idea (my opinion) how anyone could possibly think the iphone ui looks/acts better....You can alter any android phone to look almost exactly like the iphone......but there's a reason you dont see that very often, lol.
If you like simple, click this, tap that, sort of cookie cutter types of machines.....Iphone is what you want. In fact, i'd love to get my wife an Iphone if they werent made by Apple.
I'm just blown away that the iphone4 doesnt have a hotspot feature without jailbreaking it....hope im wrong about this, but I havent been able to find any other options (granted, I dont really care enough to look very hard)
The android platform is def. not easy to maneuver if you're not a tech geek of some sort, but that's sort of the point. It can be simple enough for the 14 year old girls to post FB messages(just ignore all the other options), or complex enough to write your own android(and Iphone) apps while you're at school, or whatever.
Good luck with your choice! and DO NOT go with Tmobile if you decide against the Iphone since they will probably be changing to AT&T within the next year or two........and I'd rather be shot in the foot than be under them.