If your opponent always has a Copper, then it is strictly worse, but Cutpurse usually whiffs once or twice in the late game even if there isn't any Copper trashing. My gut says this is more significant than the situations where I would voluntarily take a Curse and a Copper just to avoid losing $1 from the current hand, but it is hard to say. That is why I said it is usually slightly better (meaning that occasionally it would be slightly worse).