I haven't experienced any throttling, and I use 3-4 gigs each month. Is T-Mobile that much cheaper...?

It will cost you about $80 ($70 + taxes and fees) for an individual plan.

Your mileage may vary though. It is pretty good in cities. Do you travel out in the middle of nowhere much? T-Mobile might not be the best for you. When I think of satellite phones, I imagine being in a situation that would require me to call for rescue.

That's basically what I pay for AT&T, but to make the switch I'd have to break my contract.

The big promise is that you don't have a contract anymore. Perhaps if enough people switch ATT and Verizon will consider having this option as well. Additionally, since they have more spectrum per customer, their service will marginally get better with time unlike ATT or Verizon.

I quoted the $70+ figure because you use 3 - 4 GB data per month. You will still keep unlimited data without throttling.