How would it even be possible to have sex with the mermaid with the fish bottom? I don't even know if that would work. Anyway, I'd pick the human bottom.
FWIW, one way to solve this problem would be to postulate mermaids to be marine mammals (they do have a placental umbilicus and mammaries after all in most depictions...), so their bottom part would be more akin to that of a dolphin, not a fish.
This is our monthly thread for collecting arbitrarily contrived scenarios in which somebody gets tortured for 3^^^^^3 years, or an infinite number of people experience an infinite amount of sorrow, or a baby gets eaten by a shark, etc. and which might be handy to link to in one of our discussions. As everyone knows, this is the most rational and non-obnoxious way to think about incentives and disincentives.