While it's true that the elimination of contracts across the board at T-Mobile has brought the worlds of pre-paid and post-paid service closer together, some key differences still exist.
#1: Post-paid customers still need to pass a credit check. Pre-paid customers are not subject to a credit check.
#2: Post-paid customers have the option to put a down payment on a phone and then pay that phone off in interest-free installments. Pre-paid customers don't have that option and must pay for their entire device up front or bring their existing phone to the service.
#3: Post-paid customers can roam on AT&T towers (talk, text, and 2G data) when out of range of T-Mobile towers. Pre-paid customers don't have roaming included in their service.
#4: Post-paid customers receive a higher network priority than pre-paid customers in the event that the network is congested.
Note that the disadvantages of pre-paid service also apply to Solavei customers (no roaming, lower priority) on T-Mobile's network.