Financing through apple.com or tmobile

Im assuming T-mobile will actually be a contract phone where as Apple's won't be. If not then the T-mobile phone will probably be locked to the T-mobile network.

You then have to look at the total cost of the phone, Apple finance can offer 0% APR if you pay off within 10 months, does T-mobile? Look at how much APR you will be paying and also how much they are charging for the phone before APR then do the calculations to see who is charging you more.