They both appear to be the same species to me, because there are many color, and location variants.
Those from Laguna Bacalar in the Yucatan, can look very different from those in forested area rivers in Belize.
Beside that......
The species "name" synspillum, is no longer considered valid.
DNA testing has determined synspillum and melanurus are simply color and location variants of the same species, which may account for the color differences in your fish.
The reason synspillum was dropped, is because melanurus was described first in 1862 by the scientific community, the term sypillum was not described until 1935 (or there about).
Whatever name was scientifically used first, trumps the other.