It's kind of a moot point if you're in the USA. There are only 3 companies that actually own and operate cell towers: ATT, Verizon, T-Mobile. I'm speaking in terms of privacy, not coverage or quality.
Any other company just leases usage of the towers from one of them.
- PureTalk = ATT
- Mint = T-Mobile
- Cricket = ATT
Then you have T-Mobile that gobbled up a bunch of smaller companies and kept the names to trick you into thinking it's a different company (kinda like Nestle and their local bottled water companies). T-Mobile also owns Sprint and MetroPCS.