I would say the defining difference is that bluesky is not effectively a decentralized service. Federated(decentralized) social media platforms are great because it's hard for one single company or organization to censor the entire network.

For mastodon there are some big servers but all the uses are still quite distributed. For bluesky basically all users are registered on a central server run by the company bluesky, which has sole control over what's allowed and what's not. This has already been used by them to censor political content.

In theory blue sky is capable of being kinda decentralized but not really and even if you were to host your own account, they would still be able to censor you.

So in simple terms it comes down to:

• Bluesky is just another twitter that will eventually go to shit like every other network (and arguably already has)
• Mastodon is here to stay and while it might be small by comparison, it will never really die or be controlled by some central entity