As @eatyourpeasalready said the main difference is how they lay eggs. Mystery snails only lay eggs when there are 2 or more in the same tank. They lay them above the water line only, in one big mass. The nerite females lay eggs everywhere tiny white ones. Apparently, they can be hard to get off of plants but I've never kept Nerite only mystery snails(my pfp 🙂). Also I think the nerites don't have long antennae like mystery snails do.