Well, it is hard it say. But since Crunch is based of the user's attack stat then I would say crunch. Mainly because Foul Play's power is based from the opponents attack stat. So if you were facing an opponent that had a large Def but low attack (as many walls do) then the damage would be minor. And the power will also depend on the Pokemon it hits, not the one it initially targeted. So the power changes on switches, meaning it can be increased or decreased depending on what it hits. although if you do know exactly it will hit, then it can give an Edge even if the user's own attack is low.
Crunch however, will not change based on the opponent. So it doesn't change everytime the foe switches and is safer. As well as being used against special sweepers and walls to break them, using the user's attack not the opponents. Running little risk on switching.
Hope I helped ;D