Actually the mystery snail (Pomacea diffusa) are still apple snails (Ampullaridae). All apple snails are sexual. You need both a male and a female to be able to produce viable egg clutches which in diffusas, they are laid above the water line. Diffusas grow to golfball size. Another species commonly available is the Pomacea canaliculata. This one grows to baseball size. The largest is the Pomacea insularum at softball size but these are not suitable as the canaliculata complex are mildly aggressive especially when hungry. If you were hoping these species would eat algae, no, they will not. These snails prefer anything else and will not effectively eat the algae at all. Besides that, these are incompatible with bettas. They need hard alkaline water with high calcium level. This condition can increase the bettas' susceptibility to finrot and other bacterial infections. You can't keep the snails otherwise where the shells will erode severely. If you want snails, look into ramshorns (Planorbidae) or nerite snails (which require constant supply of algae as they will ignore anything else).
I heard mystery snails are not asexual, but I know some of them can get big, like the apple snail (like golf ball sized), which they can get confused with. I dunno whether I'd want a bigger snail, but I think a lot of people would find it neat when they look in a tank and see a big snail. A bristlenose would be a different interesting fish too, although I admit to me their face is one that isn't the most attractive, lol.