[Dain]

Sauron is probably the most powerful of the maia--all maia are not equal. Morgoth was the most powerful of the Valar because he used all of his thougt and power to increase his power, while he was also the most powerful to begin with. Sauron learned a lot from his old master, and in fact did much of the work for Morgoth (breeding Orcs, preparing his armies, etc.), and somewhere I think Tolkien even said "Sauron at the end of the third age could be considered as powerful as Morgoth at the end of the first" or something like that (in a letter or note, not text). Sauron has raised himself above the level of many of the "simpler" maiar spirits, among which I count balrogs. But, even if he is more powerful than a balrog, I don't think he could easily get Durin's Bane to join his armies, and he knows it can serve a purpose where it is, anyway.