>>8813798A Ditto would have an easier time, I would think, especially if it would have to hold onto a shape for a longer amount of time (hey, it does do that for a living). I can't imagine a Grimer would be able to hold onto any shape for very long, rather, like liquid it would more likely take the shape of whatever it can pour into. A Grimer I believe would sort of be able to make some shapes but they would be few and short lived.
Say for example, if both were employed to work with Officer Jenny's S.W.A.T. team and both were tasked to undo a lock on a door, Ditto would make a mold of the lock to form a makeshift key and a Grimer would have a better time flooding the lock until it gave way. Pros and cons, you know? It's about rocking what you've got.