One of which is the main villain of both stories, The Dark One and Sauron. Both possessed massive armies of non-human creatures the love nothing but to hurt and kill. Both aquired human servants to subvert the lands they were targeting. And both possessed great and terrible supernatural powers.
But there are considerable differences, one in particular is that the Dark One is a completely evil, inhuman, spiritual entity while Sauron was a spiritual, corrupted entity. For all of his evil, Sauron possessed human-like qualities. He was not omnipotent or omniscient. The Dark One appears to be both.
The point of this all is tell you who I think is the better villain. That would be Sauron. And that would be because of his human-like qualities. For all of his power, Sauron is flawed in some way making him more interesting, and in a way, more frightening.