Both have a similar power set. Who wins?
Back in Slapstick's original series, he got his weird indestructible molecules, the gloves to control it, and ability to access extradimensional storage space by having a bizarre accident involving a different dimension. Real toon force (the kind of abilities shown by Bugs Bunny, Wile E. Coyote, etc.) is purely inexplicable. The Mask is often shown as though he's using toon force, but his powers are actually derived mystically from the Norse God Loki.
I can see why someone would make this battle since both Slapstick and the Mask's powers superficially appear to be toon force, but that has more to do with the imagninations of the characters, not the true nature of their powers. The Mask, though, is a few exponential levels of power higher than Slapstick, who is just a very durable street level character.