That better be a PRETTY BIG bucket!! ;)
Well, all things considered, if you were to have Sauron vs. a Balrog, I think I would have to put my money on Sauron -- especially if he has The One Ring, or if it was before he created the Ring and put most of his strength into it. Something Thorin said earlier convinced me -- the part about the Balrogs being basically tied to the element of fire. Meanwhile Sauron is like pure "black magic" (for lack of a better term), and he probably has some measure of control over ALL the elements. And who knows WHAT other tricks & spells he might have up his sleeve.
Unfortunately we never really get to see much of Sauron "in action", except when he turns into a werewolf and fights Huan, so we don't really know all that he may be capable of. But think about this: when he occupies Dol Guldur, a shadow falls over half of Mirkwood! That's some pretty intense evil. And this is AFTER losing the Ring! If a Balrog was that strong, then I would think the Balrog of Moria's presence would easily be felt in Eregion.
Of course, I'm equating "evil presence" with "possible power" which might not be a valid comparison...but like I said we don't really have much to go on, as far as combat abilities. I'm assuming that someone like Sauron, whose evil can be felt hundreds of miles away, would be a pretty terrible foe to face in battle.