Culture wise I’d say that Bluesky feels a lot more like Twitter immediately before Musk took over - there’s lots of shitposting & also a lot of “look at me I’m so irreverent so please follow me” vibe to it. It’s easier to find people to follow but harder to find interesting content (on mastodon you can rely on hashtags). It’s also a lot harder to get engagement if you’re not a Twitter big name, there’s a lot more parasociality going on, with more focus on individual posters than on the actual content, which IMO is exactly what a lot of people are looking for.

Mastodon users tend to feel very earnest to a fault. The whole “you forgot to add a CW to your food pic you bastard” thing is unfortunately real, but I’ve mostly stopped seeing this since I put a warning on what I will and will not CW on my timeline. On the bright side, there’s a lot more friendly engagement from strangers. Politically, mastodon users as a whole lean left with a lot of socialists/anarchists but you can curate what you see. It also depends on what circles you follow, for example, I (as a Chinese speaker) stumbled on a huge community of Chinese users who are feminist and anti-CCP, but will probably not consider themselves leftists.

As a whole, I’ve mostly abandoned Bluesky in favor of Mastodon but that’s a personal preference - both are unlikely to completely replace Twitter & the landscape will likely remain fractured for a very long time so just choose the one that vibes with you.