T-Mobile
Alex, Do you have a T-Mobile data plan or a current phone using T-Mobile that has a data plan?
If not, meaning no current Data plan, then you would have to add one, which would probably end up costing you more than if you just use the 3G version of iPad with monthly service from ATT.
Not like if you had an existing device like an iPhone, unlock it to use your T-Mobile SIM just for phone use, either way one device is "disabled" not being able to use data, cellular or both while the SIM is being used in the other device.
Whereas, I have three iPhones currently for different jobs, one uses the regular ATT sim with 3G and data, a second with unlocked/jailbroken to make use of one of my contractor jobs service with T-Mobile, but they don't have data on that SIM and the other I just use for WiFi when I am too busy with one of the otherphones or almost like multitasking between devices.
Typically though I am making use of just the two with cell data and the other as a standby in case another contractor has a different carrier line. It would be awesome if I could just use all three SIMS in one unlocked iPhone, which I have seen devices which enable use of two, but not three at the same time.
I'm just trying to get your reasoning for using the T-Mobile SIM.