I would give them the ability to do both. I've always suspected The Dark One was faerie. So there is no reason he shouldn't be able to step into both worlds when the dust settles. He did that everywhere else just fine. Just needs to establish a doorway. After all, there is always need for his services in both realms...

And Belle could be the good cop to his bad cop. Got a rep to maintain after all!!

*HUGS*