They both have some pretty.. 'reguar' designs, but I think a bee with drills for arms wins over a generic butterfly anyday. So Beedrill wins in terms of design.
Battle-wise, up until Generation V they were both the same to me. You know, until Quiver Dance hit Bug-Types and everything else in the game. Butterfree wins.
They're both great, but I think I'll have to go with Butterfree on this one. It has a design as well as a refreshed usage in battle.
tl;dr - Butterfree