What T-Mobile smart-phone is better?

I was deciding if the I-phone 3gs was a good choice, but i was thinking if a T-Mobile one was better. Please help me!

I love the I phone. Sprint is supposed to get the I phone 5 in October. T Mobile is merging with att so it is not a good idea to sign a contract with them. There are many complaints with the T Mobile phones.I hope that this helps.