Mitchell Johnson is, at his best, infinitely more damaging than Stuart Broad. However, he is also fairly inconsistent (being a lower-order batsman) and relies on power and hand-eye coordination, rather than a sound technique. He is roughly comparable to Lance Klusener (only Klusener was obviously a much better batsman and a vastly inferior bowler for all but one Test). As his ODI record shows, he's a nervous starter, too.
Broad is more technically adept, more consistent and has more batting potential (if Geoff Boycott is to be believed), but lacks the ability to take the game away from sides in the manner that Johnson can. Despite averaging over 30, he's only scored 3 50's. That's indicative of a player who gets starts, but who cannot go on with them - the exact opposite of Johnson, exactly, who seemingly either scores big or scores relatively little.