it simply depends on the situation.....
If there are passed pawns (for both sides) i'd say Rook+ bishop is better...
simply because the side with a queen hasn't anything to sacrifice - whereas - r+b I could sac a Bishop (preventing promotion) while still have a rook on the board.
also A queen is a very good at picking peices off - the the queen can repeatedly check it won't be long before you start losing the odd pawn or worse.