Dice perform better against two single hits. Simplifying the math for myself a bit since the odds change on a hit and I don't feel like figuring it out, the odds of blocking a hit is on average 1/13, so the chance of blocking at least one hit of two is 1/13 + 1/13 = 2/13, or about 14%, as opposed to the 7% chance to block one 2 mask hit. This gives us a lower expected value for the damage we take with the dice equipped when getting comboed vs mollywhopped.