I agree with Laki!
In my experience Mystery snails are too vulnerable to betta attacks. I had one once that got its eye stalk bitten off by my betta. I recently got some nerite snails, though, and they don't really come out far enough from their shells to be damaged by a betta. And because they mostly look like a gliding rock from a betta's POV, they are less likely to attack it. My girl pecked at hers once, and stared it down a couple times, but they pretty much ignore the snails now. Nerites are really good cleaners, too. So that's a plus. Although one of mine is a great cleaner and the other one isn't. One of them cleaned all the brown algae off all the surfaces in his section, and the other snail just makes skinny trails through the brown algae. lol