A t mobile sim from ST?
I first got the ATT sim when I signed up and I also wanted to see what the coverage for T mobile was like. So I ordered the T mobile sim from ST. When I got the SIM I called ST and had them transfer my service over to the sim card. You dont have to pay for another month or anything like that as your current service gets moved over.
NOW, if you find out that you dont like the T mobile sim card you will have to buy another ATT sim card from ST as you cant reuse the old one.
My suggestion is to check T mobile's coverage map and it is very accurate. As long as your in the "good" area you will have great service with data speeds around 3-5mbps.
T mobile is definitely the better option as far as service and data speeds. Data speeds on T mobile are between 12-15 in excellent coverage where as ATT is around 5mbps in excellent coverage. So if you can, get the T mobile sim card. It's way better.
Hope this helps.