Postscript: It’s time to bring some reason into the discussion..
Just because the mermaid has a mouth doesn’t mean she is going to let you put something in it. Chance are though the seduction will be easy like in the tom hanks movie and you will get a blowjob here or there. But I don’t think it will be blowjobs on demand like most men are imagining. How about if she doesn’t have blowjob skill?
For the reverse mermaid, it is true that you have vagina and anus and since it’s controlled by a fish brain you can have access to it all you want through rape or whatever. But keep in mind when you have sex you have to hold onto her scaly skin. And she probably will smell like fish. You wouldn’t want to hit that everyday… though from the back maybe it’s easier.
The key point no one mentioned is that you will get very lonely. Assuming the mermaid speaks your language, you can keep yourself busy with her and maybe even develop something serious.
I pick A.