Yesterday I drove from Carbondale IL to Chicago and had no service or calls only service with T-Mobile about 80% of the time. My friend with Verizon had full LTE coverage the entire way. There's more to compare than just the unlimited data package.
I drove through the mountains of northeast and north Georgia on Monday to see the solar eclipse. My previous experience with AT&T in the mountains was bad. I switched to T-Mobile from AT&T in February 2017 due to AT&T wishing to rip me off while I was about to travel to Canada.
I love T-Mobile. While in Canada, on two occasions now (both are 10-day long) I had amazing coverage with LTE and full speed tethering everywhere. First time I was in Toronto and second time in Quebec's most remote areas. Never one time was I without signal.
I suspected that T-Mobile would be bad in mountains of Georgia and got a prepaid Verizon SIM for $40 with a 30-day coverage and 3 GB data just in case. During the eclipse, I had a similar coverage between Verizon and T-Mobile. (I had two iPhones so I could compare in real time.). On the way back I got in so much traffic that we sat for hours barely moving. Both carriers showed strong signal but data was unusable on both carriers. Verizon phone couldn't make calls either - all circuits are busy message. T-Mobile was able to make calls with good quality. Then, I got out of traffic by turning west and drove through high-elevation mountain roads. I spent 2.5 hours going through the mountains. At times both phones lost signal completely. Most of the time, both phones had signal. There were a few times when Verizon had signal but T-Mobile didn't - maybe a mile or two of the road. However, when that happened, Verizon's signal didn't work with data.
So, here are my conclusions from this unscientific and empirical test. Verizon still has better coverage than T-Mobile in rugged US areas but just barely. Where T-Mobile has coverage, the LTE data speeds are significantly better than those of Verizon - sometimes by an order of multitude. Verizon doesn't seem to have as much density in their capacity as T-Mobile does. When thousands of devices end up communicating with the same tower, Verizon's ability to connect calls melts down way before the same happens with T-Mobile.
I've already traveled with my T-Mobile phones through rural Alabama, rural Panhandle Florida and stayed on Santa Rosa Island (Pensacila Beach). T-Mobile works all the way there even off the beaten path in rural Alabama and Florida. T-Mobile works everywhere in Atlanta metro.
So far I am extremely happy with T-Mobile. I am still a little leery going to the mountains in the US with T-Mobile, but everywhere else (including Canada), T-Mobile is phenomenal. And did I say that the coverage in Canada is amazing and costs absolutely nothing additional on top of my regular monthly service? I'm paying $100 including all fees and taxes for 3 lines. Each line has 31 GB of unthrottled data. How cool is this? T-Mobile is the only carrier that's doing things right in the US.