My opinion:
Great Maul for 21/18 build.
Mallet for 24/15, 27/12 and 18/21
Maul for 30/9, upwards and 15/24
You will almost never crush when you don't hold the attack, but you're almost certain to crush if you hold it for half a second or so, depending on the weight of your opponent mostly. Don't expect to crush an opponent's great maul for example.