Both
mercatoris and
joubini are nocturnal Caribbean dwarfs. Historically is it said that they are hard to distinguish but I don't believe this is true if observed for a short time in captivity. Since both are nocturnal so seeing them at all in the wild is rare and they are usually by-catch of either fishermen (crabbers) or live rock farmers.
Mercatoris is a large egged species and a few of the young can be raised in captivity.
Joubini is a small egged species so there is no current success with raising any of the hatchlings. At one time (not that long ago, 1960's I think) it was thought that there was only dwarf, joubini, in the Caribbean so all captured dwarfs were labled
joubini and it has taken a long time to negate the only one dwarf species literature (that is still found in credible papers doing searches on Caribbean dwarfs since they don't update the papers as new discoveries are made).
As far as your concerns on which octopus you get, keep in mind that they don't live very long so any animal that is physically suitable for the tank (ie not briareus) can be a great octo keeping experience. Unlike dogs, cats, birds and most other pets, you have to learn to accept that the octopus you keep will only be with you a short time. A goal should be to keep it alive and healthy for at least its natural life span and learn about the animal, hopefully to the benefit of the next one and share what you learn.
Tom actually took a brief in keeping an octopus when he lived in the north (years ago).