They are two different types of software that can access the fediverse. Check out lemmy.ml and kbin.social for instance to see the differences. They can both talk to one another and see posts from each type of software. (Kbin can also see posts from mastodon which is kind of like Twitter) I’d say check both out and see what interface you like better. Since they both can share posts with one another you really aren’t missing much by choosing one over the other.