I recently left TMobile and got Verizon. the biggest factor was price. for me Verizon was cheaper.
now that I'm on Verizon I can also see how much more reliable they are. i have a Verizon phone that I used on TMobile and I was beginning to think something was wrong with the phone. with T-Mobile I would have data outages that lasted a few days, texts that went out for hours at a time (both sending and receiving issues). TMobile coverage was good and I was overall happy but coming back to Verizon I really realized the network issues I was having with T-Mobile.
with a Verizon sim the phone works fine. with an att gophone sim, the phone works fine. with a TMobile sim it had issues and that was with full coverage band 4. even with the band 12 phone the issues were there. I think congestion was a big factor.
what I've learned
1)if you want rural coverage either Verizon or at&t is your best bet
2)TMobile with having just one postpaid plan choice felt restrictive
3)for me both Verizon and at&t were cheaper unless I needed tons of data.
it's not a knock to T-Mobile since I was pretty happy most of the time especially with coverage but I see now how much more reliable at&t and Verizon are on a daily basis.
to sum it up. why Verizon? cheaper and more reliable. cheaper is what really got me to make a move so the reliability is a nice plus.
just this morning a friend called me on his home phone since his TMobile line is out. not sure why TMobile is having so many issues lately but even Verizon outages seem to be fixed fast