Beaver seems to be open source, although I cannot find any link to the code nor license (although the "Terms" page mentions MIT: https://www.beavernotes.com/Terms.html)

Obsidian isn't open source. But, it stores your files are normal markdown files (cannot see if Beaver does the same) so it's easy to import/export if needed, and also have a huge number of plugins for basically anything out there.

Obsidian is also pretty established (read: "old") at this point so the ecosystem is already big, stability is good and it's mature enough to be used professionally. Probably Beaver doesn't come close to that (yet).