Sounds like you've noticed that AT&T is usually the one that works when Verizon and T-Mobile don't. But hey, T-Mobile kicks ass when it comes to speed and it works in most places. So even though AT&T is the most reliable in those tricky spots, T-Mobile wins the race in terms of speed and overall performance.
You've got the right idea by using your postpaid Verizon phones on Wi-Fi when their service sucks. That way, you can enjoy the perks of T-Mobile's speedy connection without compromising on coverage.