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Route

In order to establish what is more likely, you would need to have a rough idea of the evolutionary route they took. Thinking about this may lead you to a history that explains how they evolved naturally to look as they do, or you may decide natural evolution is unlikely to settle on that shape, and therefore seek another explanation.

The main question is, are these fish that have evolved human aspects, or humans who have evolved fish aspects? If they started as fish, there is no reason to expect the tail to change from vertical to horizontal. If they started as humans, we already know that an evolutionary path exists from mammal legs to a horizontal tail.

Reproduction

Evolution is about reproduction. Is the reproductive system fish or mammal? For fish, with external fertilisation, a vertical tail is not a problem, but for humans, with internal fertilisation, a horizontal tail is more compatible with the body structure that led to this. It isn't impossible for internal fertilisation to evolve with a vertical tail, but it seems more plausible with a horizontal tail. For a human evolving into a merhuman, that method of reproduction would likely provide a barrier to evolution towards a vertical tail.

Alternatives to evolution

A sharp distinction between one body type and another seems unlikely to evolve naturally. If scales are beneficial why wouldn't they appear all over? A real world example of creatures with such a sharp distinction is biological chimeras. These are creatures composed of genetically distinct cells. This happens naturally in many animals (including humans) resulting in parts of the body having different genetics to others. It can also occur between species, although not naturally as far as we know. Goat/sheep chimeras have been created (called "geep"). However, these do not have a neat line across the middle - the divide between the two species can be anywhere and may not be visible externally. They are more likely to appear "patchwork" than neatly half and half:

Also the offspring will be of the same species as the reproductive organs (either a sheep or a goat, not a geep). There are species where the majority of offspring are chimeras, for example the marmoset. It's difficult to imagine how an inter-species chimera could give rise to similar offspring. It's also unlikely a chimera would survive between two such different species as humans and whatever species of fish were used. The fact that fish use external fertilisation does provide the possibility of a fertilised fish egg being introduced to a human's womb, to then combine with a fertilised human egg. If merpeople had two sets of reproductive organs then the fish and human eggs could be both released internally, and then combine following fertilisation on the way to the womb. In this unlikely set up, I would expect the tail to be vertical since it is produced directly from fish genetics.