Beedrill - gets STAB on two physical types (though it doesn't learn any good bug moves save HP bug), okay-ish physical movepool, average attack and slightly better speed. Butterfree has average Sp Atk and horrible attack and gets no STAB on any special types, so it's better as an annoy-er, but gets killed too easily to be much of one - any half-decent rock move will 1HKO it. And ingame I find sweepers are a lot beter than annoy-ers, they make the game go along a lot faster and such =)