Hmm. This may be an unpopular opinion but I guess I don't really care. I love my AT&T plan since I'm still grandfathered in for unlimited data. I'm not going to walk away from it, especially not when I'm just part of a bunch of anonymous statistics.
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.