You cannot really compare them. Mastodon is similar to Twitter and nothing more, it’s a microblogging service. Lemmy is like Reddit or the Usenet from back in the days with a nice threaded view. Many people on Reddit never used Twitter and vice versa.
This new fancy stuff is all part of the Fediverse, but that‘s a bit misleading. You can use one part and completely ignore all the other ones.
The last time I used it is about 10 years ago. Don’t miss it. Always hated the cables dangling around and pulling the headphones out of the ears. Wireless is just way more comfortable.