It also depends on *what* is between those legs. Everyone seems to assume that B has a vagina. Which might not be the case. Hey, we're not even sure about A's biological sex/gender. Sure, they have boobs, but that doesn't mean anything. (trust me.)
Don't fish use the same hole for both reproduction and excretion? That can be unpractical, unless you're into that. Now that I think about it, fish reproduce by spawning, so unless you have a thing for eggs/sperm being sprayed over any of your body parts, don't hope to get too frisky.
Fish eggs are fertilized outside of the body (no penetration), so the hole probably isn't big enough to do anything with. And if there's scales around it you'd probably just...injure yourself if you tried.
I'd still be more attracted to just kissing mermaid A than having actual sex with mermaid B.
Don't ask me how I know this, but most of the time people draw mermaids with having a human vagina at the exact point the human and fish sides meet, so unless this artist really wants us all to die, that'd probably be the case here. I'd take, as I said twice now, Mermaid A.
They're mermaids. Given they spend a lot of time underwater, they'd have developed a way to protect their skin. Mucus or a slight layer of scales are plausible options. Either way, body hair is unlikely for both.
wait if i was going to give a serious answer i feel like being able to have meaningful social communication is a bigger priority than a specific kind of sexual intercourse. i mean otherwise it's gonna be pretty boring.
That's the problem: most people think "companion" means "sexual partner". I say, those youngsters... Me? I'm looking at the problem from a practical angle. Who could be the best partner for survival on a desert island. (Both have their advantages, but A adds social interaction.)
Did you think about asking for consent before any kind of sexual activity? And don't forget protection. Whatever their genitals and species, you don't want any fishy diseases.
That's something again: which will feed you more, someone who can help you with hunting/fishing or farming, or a single mercreature carcass they can provide meat for a couple days, a week if you manage to preserve it? I'd need to know how much meat a human body and a fish can give respectively, then determine whether A or B have the most edible meat. I'd need their age, height and weight first, as well as their skeletal designs and organs. Do mermaids have human organs in the upper half and fish ones in the lower half, or is it a combination? Combination seems plausible, because having gills on your hips is likely unpractical. Look at insects, they breathe through their abdomen and that's why they're so small: because it's hard. Again, of the oxygen rates in the air were to ride, insects could breathe easier and reach bigger sizes. That's why mermaids can't breathe by their hips, as there isn't enough oxygen in the water. ... What was my point again?
A would be better to catch fish. Assuming she doesn't think that catching versions of herself that don't have human parts is kind of creepy, give her a net and let her go to town for a few hours. Of course, you'd need to be her friend first; chat it up, do favors for her, get intimate, know her better, and then you ask for her to just get A SHITTON OF FISH. More lucrative than just having a one time big feast. What about the future, friend? ...Or you could just get a whip and make her your fishing slave.