Forgive my ramblings, but here’s the main differences I see, from a community perspective:
Bluesky’s for people who loved twitter circa 2015
Mastodon’s for people who loved the format but hated the way the platform made use of it. The community is FOSS-focused and anti-corporate.
Bluesky folks are anti-corporate, but they still want their social media to be on a single platform and tend to dislike federation
Mastodon folks tend to be in smaller circles and more tech enthused
Features-wise, Mastodon kills the algorithm in favour of chronological timelines and lists, while Bluesky embraces algorithms, allowing people to even make their own algorithms for the platform. Bluesky’s AT Proto uses “DIDs” to identify users, which are associated directly with a domain^[or subdomain]. This means that when federation does eventually happen, usernames will just be @my.domain.com instead of ActivityPub’s @actor.
Federation’s still not enabled so I have no clue how things will look and feel on that front, nor am I familiar enough with the protocol to make any claim about how versatile it is. ActivityPub is flexible enough to be a Twitter clone, a reddit clone, a blogging platform, a youtube clone, a twitch clone, a goodreads clone, or several other formats. AT Proto’s currently only proven to work for a Twitter clone.