I tend to go with the FR take, whereas mountain dwarves are a different ethnic group splintered from the hill dwarves “stock” that remained in the south.
In a more pragmatic way, hill dwarves and mountain dwarves are simply two variants on the same race, offering different options during character creation in a typical D&D way of selling races and classes as packages. Basically, do you want dwarf A or dwarf B? These are the two choices you have.
Why aren’t humans also offered in a variety of ethnic/cultural/geographical variations? Boy would I ever wish they were, but I’m in a minority it seems.
With floating ability score adjustments and “pick-a-feature” race design, the distinction becomes less and less relevant. I’m still trying to figure out whether it’s a good thing or not…