I used to have T-Mobile. I was never able to use a lot of data because it simply didn’t work in several places I needed to have it. Their coverage maps are a lie.
Here in northern California, T-Mobile works very well—I’ve seen 95 megabits/second download speeds (pretty much the max) on my iPhone 6) in most metro areas. On my old Samsung Galaxy SIII, I was getting around 30 megabits/second max due to the fact the phone only used GSM/HSDPA cellular data.