I have AT&T and T-Mobile (work and personal cell phones) and have had iPhones and Samsung Galaxy S7 on AT&T and iPhones, Samsung Note 3 and OnePlus One on T-Mobile. The phone itself makes a huge difference. My wife had an Iphone 5S on T-Mo in B/CS and had dropouts a good bit. She switched to a 6S with Band 12 and gets great coverage, better many times than my AT&T iPhone 6, in the same areas. My OPO had decent coverage, but it could be spotty in the north end of town. My S7 on AT&T and my Note 3 on T-Mo both had pretty damned good coverage all over.
I would also note that my T-Mobile service has been blazingly faster than my AT&T service. AT&T gets me around 40/1.5 and my T-Mobile has been getting around 85/10 lately... Both are perfectly acceptable for use though
My wife also travels between here and Arlington quite a bit, and the older iPhone 5S without Band 12 did have a good number of connection drops along the way. Since upgrading, she hasn't noticed hardly any.