I was on Mastodon starting soon after Musk bought twitter, and bluesky few the last few months. Mastodon is a lot more similar to lemmy in terms of being part of the fediverse, while bluesky sort of claims to be going for a federated thing of its own but so far is pretty much just a centralized server like twitter so far. Perhaps because of that, I find bluesky a lot more engaging and more similar to pre-Musk twitter. A lot of the choices the Mastodon creators implemented were deliberately reducing virality and "one big chatroom" feel that I at least liked about twitter. I wasn't looking for those changes, just a microblogging social network not owned by fascists.

So basically if you want something like lemmy go with Mastodon, if you want something old twitter go with bluesky.