I would say Android. The iPhone is a good toy and all. Apple fans will obviously defend the iPhone till they croak. But Android is made by Google. Google has a very open stance on things. Not just that, but you have multiple manufacturers to choose from, all with different feature sets. Also, multitasking on an Android handset is easy. I find myself streaming Pandora, Last.fm and Slacker all day long. While I'm listening, I might hop into facebook, or surf the web without interrupting my music. Also, everything's interated. My contacts show me their facebook status. My google searches bring up things close to my location with the GPS, and music on my SD card brings up artist details and similar streaming stations with last.fm. It's awesome.
Another point...T-Mo's 3G connection is different from AT&Ts. So the iPhone would only work on the slower EDGE connection. Even though T-Mobile's 3G is sparse, it's still nice to have it if it's there.
In the end, if you like Apple, and iTunes, the iPhone might be your thing. But if you want something different, but very, very capable, go with Android. BTW...free GPS doesn't hurt either if you get a 1.6 version handset. But all Android phones will have it soon enough.
Another point...T-Mo's 3G connection is different from AT&Ts. So the iPhone would only work on the slower EDGE connection. Even though T-Mobile's 3G is sparse, it's still nice to have it if it's there.
In the end, if you like Apple, and iTunes, the iPhone might be your thing. But if you want something different, but very, very capable, go with Android. BTW...free GPS doesn't hurt either if you get a 1.6 version handset. But all Android phones will have it soon enough.