Even if your handset is capable of all the major networks, if reception has been good on Airvoice? Stick with another carrier on the same network.
I've been with US Mobile in the past on the T-Mo end, but needed to migrate to an AT&T MVNO a couple years back due to reception issues and a desire to avoid having to purchase a Verizon handset just to stay with them and fix the problem. They were fine, but again, if you've had good coverage with AT&T MVNOs, stick with the critter you know.
I'm with Red Pocket now, and they have plans on AT&T, T-Mobile/Sprint and Verizon. Definitely stay away from the Sprint plans and MVNOs long term as a general rule, the network is slowly getting dismantled and the spectrum re-farmed to T-Mo 4/5G outside of their existing LTE towers that aren't redundantly placed to T-Mo's existing infrastructure. Not a big problem with the Sprint postpaid customers long term, but very problematic for the non-roaming MVNOs. Also, read the terms and opt out of Red Pocket's data collection if you go with them specifically and don't like that sort of thing. They're perfectly cromulent, otherwise.
There's also Puretalk USA which is another dedicated AT&T MVNO.
If you're up for a bit of a read,
I just posted a bit in another thread that's directly related to Mint Mobile and their business model, even if it doesn't seem like it immediately. Take it for what it's worth.