I use both and much prefer Mastodon. As others have said, it’s more mature and the apps (of which there are many) are more polished. Bluesky is a bit glitchy and less feature rich. In fact, you still can’t mute phrases on Bluesky, which is a deal breaker for me.
Mastodon is harder to find people due to the federated nature, although some mobile apps such as Megalodon on Android and Ice Cubes on iOS do pull follower and following lists from remote instances which makes it easier to find people.
Bluesky is more like old Twitter, and many people who I’ve followed for years are on there. Trying to out quip each other. Mastodon is more friendly and I’ve found lots of interesting people to follow.