I haven't experienced any throttling, and I use 3-4 gigs each month. Is T-Mobile that much cheaper...?
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.
I quoted the $70+ figure because you use 3 - 4 GB data per month. You will still keep unlimited data without throttling.