@Arachnid I use and enjoy both. Both have their strengths. My opinion:
Mastodon is actually decentralized and mostly community-driven, not dependent on a single company. But due to this there are some limitations, like never getting the full picture (not seeing all the replies to popualr posts) or being at risk that if your server goes down, so does your profile and data unless you manage to migrate before. BTW Vivaldi instance is pretty cool, no excessive blocking of edgy humor, but still pretty well protects its users from spambots and scambots. Mastodon also has a vibrant community creating custom UIs, plugins and accessory software. My personal favorite is the Phanpy Social UI that makes the site way easier to use, less confusing and IMO more beautiful. Culturally, Mastodon is full of huge nerds. Lots of free software activists, cyberserurity experts, developers etc. The mood over there is pretty anti-corporate and hacker-positive.
Bluesky has open architecture and some decentralized features, but is mostly centralized. Due to this it's less confusing for regular users and a lost profile can be recovered. But this also makes it succeptible to future enshi**ification (they have taken big investments from VC funds and still have not fully revealed their profitability plans). Compared to Twitter, Facebook etc. it allows a great deal of customization of the content you see, there are community created starter packs, blocklists and custom feeds that make the onboarding really easy after migration. Compared to Mastodon it is IMO just way more fun to use and explore. Culturally it's basically the center-left part of Twitter, lots of indie artists, scientists, more and more journalists and comedians.
TL:DR: Mastodon is a very nerdy near-ideal of what a decentralized social network should look like. Bluesky is a fun, left-leaning and customizable Twitter alternative.
BTW the two networks are bridged over by BridgyFed. It's opt-in, so not too many accounts are bridged RN, but those who are, can be followed and interacted with from both sides.