I had T-Mobile, briefly, before I realized it just wasn't going to work for our family, a lot of our winter travel routes (skiing and various other activities, use cell tethering going to-from a lot) were basically not covered. We went back to AT&T, which has good rural coverage in WA state (not excellent like Verizon, but good enough, mostly).
When I had T-Mobile, my 950XL was insanely fast, in the Seattle Metro area, sometimes pulling in excess of 50 Mbps. With AT&T again, I'm usually luck to get something like 15/5, maybe 18-20/7-10, on a great day. Some places are better, but T-Mobile in-town was definitely a lot faster, on average, and this was back in the .218 days (I went back and checked, the whole month we were on them was mostly .218 Threshold). So you can have great speed experiences, on Threshold, and T-Mobile, with the right location. What is that they say "location, location, location..." ;-]
YMMV, obviously. If you can't ever get decent speeds, I'd be wondering about the hardware too.
The 950 single SIM has a cat6 radio, and the dual has a cat4, but in practical terms, it probably doesn't matter, since cat4 is limited to 150 vs the 300. I've yet to see much above 50, or really care about it, at this point. Maybe I'm accepting mediocrity, or maybe I'm just accepting, I don't know, most would consider me pretty demanding of hardware though.
I really want WiFi calling/texting, more than anything, I want our darn providers to flip the bit in the US, as I often am at places on the weekend where I have awesome WiFi and zero cell...