Well, depends. Are you looking to sign a new contract? If so, with you? Are you leaning one way?
What do you want your phone to be able to do? Depends on what your needs are.
If you want to go Android, don't want to sign a new contract, perhaps look at this
site and buy an unlocked phone. Then you can go with whoever you want and are not stuck in some long term contract. Depends on the money you have as well I guess.