This is a funny question. Sorry, this is not my field of expertise...
I wonder how many other hobbies talk about poop....?
If I had to venture a guess, I'd say that snails don't eat much so they probably have very little poop, which would make their poop inconsiquential compared to the quantities a fish can poop out.
Furthermore, it would probably depend on the type of fish or snail. As an example, is the fish a meat-eater or more of a herbivor. I would imagine that a meat eater would produce higher energy poop compared to a herbivor. The same goes for the snail, does it graze on algae or eat detritus?
My guess is that something like a nassarius snail, whose diet already consists of eating poop, would produce super high-energy poop, which would make them super-poopers.
Poop.