Correct me if I'm wrong, but different target users here :) Beets seems to be for consumers who want to organize their music collection, and while Mixxx can help with that too, it's mainly for music connoisseurs who play for others (AKA DJs). I'm not sure how well someone looking for the first could use Mixxx, the interface isn't exactly the epitome of user-friendliness if you just wanna organize then press play :)
Mixxx is best used with a DJ console, however for me it is also easily used as a player that automatically picks a song from some selected crates in Auto DJ mode.
So I'm in the mood for Salsa, I put that crate as the AutoDJ source.
Later I can change it to progressive rock, or 90's disco, and it will randomly pick songs from the selected crate or crates.
I can also do DJ things, like looping some sections or mashup some songs.
