Biggest difference that I noticed is in Bkueaky you identity is not tied to the site you signed up at.

Mastodon has a huge flaw in that your identity is not stable. Like email your identity is tied to your provider, and if you change providers you have to change your identity, and all your contacts have to update their addresses boom so to speak.

In Bluesky your identity is stable, with some caveats. It's easy to "bring your own domain" without having to host your own service provider.