Beerus wins. He is way stronger than Buu, who can destroy the universe along with the Kaioshin realm.
Beerus is much faster, can turn him into sand or seal him away.
Kirby can crack planets (in Super Star and Amazing Mirror), but cannot absorb enemies stronger than him (like bosses). His feats are mostly inconsistent and used for laughs in his universe. Also, his reaction and strength are shown in the sub games "samurai kirby" and "quick draw", where he is mostly on par with regular enemies and bosses.
Buu is way too fast, strong and has better attacks!
I'm just worried that we won't see it released in English for a while, especially because the 2008 DBZ special didn't even get localized.