As someone who's company has paid for his phone/plan for the last 15 years, we are in the middle of having our numbers released and will be in charge of selecting our own provider and coverage plans. We've had Verizon for years, and I haven't had any complaints with coverage (mostly suburban/metro area), although I'm probably in that low-hanging fruit area where everyone has coverage.
For me now, the biggest interest is in unlimited data, tethering/hotspot usage, and how much I get before I get dropped to lower speeds, although 10 GB for hotspots seems a default. AT&T slows speeds after 22GB of data; T-Mobile seems like they don't except if you fall into the >30GB a month and there is congestion (take that as you will). My new vehicle has a built in hotspot (cool or a WTF, I'm not sure yet), but I'm stuck with AT&T if I want to put it on a plan. So far T-Mobile seems to be the most cost effective, but as others have mentioned, rural coverage will be lacking. Right now, I expect I'll be negotiating with T-Mobile to see what my wife and I can get. If that is enough savings, I may then look at a separate plan for the vehicle to help offset coverage when I travel (I travel a good bit for work).
You need a matrix just to try and keep track of what you need/where, and how to connect to avoid issues or using the 'wrong' data.