If you buy the phone from Apple than you pay Apple if you buy it from TMobile then you pay T-Mobile. these are not interchangeable so if you buy it from Apple it will not be on the TMobile Bill and vice versa.
And also, purchasing it from Apple will avoid having a SIM-Lock applied. THen, it's hassle free.
Also, since it's not sold by T-Mobile, T-Mobile can't apply a SIM-Lock to your phone in the future.
Yes, you heard right, they can do that since they had the SIM-Lock applied in the past.
Purchasing your iPhone from Apple is always the best solution, unless you get a really good deal from T-Mobile.
But this is true, the payment is not going to the bill if you purchase from Apple. You pay Apple, or you setup a credit with them with Monthly payment.