I would love a combined T-Mobile/Sprint.
While their coverage and lack of coverage overlaps quite a bit, at least in my market and places I frequent, data speeds are the opposite between the two carriers. It seems like where I get 30-50 Mbps on T-Mobile I got 70-100 Mbps on Sprint. There is even an area where I got 200+ Mbps, but upstream was only 5 or 7, must have been a site testing CA on 41 as that is the only location I ever hit those speeds. Ironically that was in the parking lot of the plaza where the T-Mobile store is where I signed up for service. On T-Mobile where I get 250+ Mbps down, I'd only get 15 or 20 Mbps down on Sprint. My issue with Sprint was due to my proximity to Canada and IBEZ, they can't light up band 26 here. With T-Mobile's band 12 and now 71 I'm hoping for better rural coverage. When in eyesight of my closest tower I can get 100 x 20 on band 4, but I lose the signal about a mile from my house. Sprint is on the same tower, and I can barley get a signal in my bathroom on band 25, but I do get a signal.
My ideal combined mix would be T-Mobile's 2 and 4 and Sprint's 41 for speed and T-Mobile's 12 and 71 and Sprint's 26 for distance.