Two recommendations: find out what your friends and colleagues use - the ones the will be around when you have questions. If everybody in your crash pad has an Android phone , that may be enough right there. Also, find out which apps you'd like to use. These aren't really phones so much as pocket computers, so, for example, if FxCal is the key app then you want an iPhone. The apps are going to make the phone more than specs or operating system.
Carriers aren't an issue for the iPhone anymore; I heard today that even Cellular South is getting the iPhone, too. All iPhone 4S are CDMA and GSM, so even a Sprint iPhone should be able to accept a foreign SIM card.