Letterman's monologue is painfully bad. His interviews used to be worth watching but now are going downhill. He uses the same schtick EVERY time where he just repeats what the guest said 2 minutes before in a different context. When he riffs behind the desk about nonsense between the opening and the guest he's good.
Craig riffs about nonsense non-stop and is usually pretty funny. It's just plain more enjoyable to watch, since it's so loose, compared to stiff show that came before it.