Which branch becomes the sub-species in that case? The one with the smaller Population? Or the one that diverged more obviously from the nearest common ancestor? Or the cultural less dominant one?
Or perhaps the one that traveled, as opposed to the one that stayed put in the area where it still remains dominant? That'd make the ridgeheaded Romulans a subspecies, and Vulcans the mother species, even if the Mintakan doubletalk suggests the ridges are the "more original" phenotype.
Ultimately, I guess "subspecies" and "race" and "breed" are interchangeable, but only technically: politics and fashion dictate which is the applicable term at a given time and place.