I would keep that T-Mobile Sim card and try it for a while before you plunge head-long into a AT&T contract. I am an AT&T customer but frequently activate a T-Mobile Sim I have on the $30 plan. With that I have noticed that the $30 plan provides LTE and it is popping up all over the place (at least where I live), which is more than I can say for AT&T. AT&T's LTE foot print has been very stale in Southern California lately. There is no doubt they have better coverage than T-Mobile, but I would just keep that T-Mobile Sim and use it in the N5 before you switch. From what I have seen T-Mobile's LTE roll out is more tower by tower than city by city. You might spend a great deal of your day by one of those towers. Being a contract customer is a great big ball of hurt after being prepaid for a while.