The difference is only the width. The allplay is 8mm wider. For this reason the Allplay is still good on firm snow but it's extra width makes it equally as good in up to 10"/15cm of powder so it's more versatile on a wider variety of terrain from the groomed to the powder. More Allplay info...

The Whipit is better on firm snow because it is narrower and quicker but it has less float in the powder but if you ski mostly firm snow or park then you'll gain quickness and a very nimble feeling. More Whipit info...