[the phantom]

I agree that it depends on which dragon or balrog you're talking about. Gothmog was the baddest of the Balrogs, and Glaurung and Ancalagon were the nastiest of their kinds.
Yes it's true that a bowman killed Smaug, but that was kind of, well, lucky. Bard wouldn't have stood a chance in a "fight" with Smaug. Smaug would've torched him, knocked him a mile in the air with his tail, and the ate him. Dragons are huge, and also have spirits that are equally potent to a Balrog's spirit. Glaurung could pronounce dooms on people almost like Morgoth himself, and cast spells as well. Before Glaurung had ever reached his full might, it still took a whole mob of archers shooting at him to finally make him turn tail and run, but only after he'd done a lot of damage.
Also, as someone pointed out already, when the flying dragons were released, they even drove back the forces of the Valar.
I'm not saying that dragons are more powerful, but I do think that the voting shouldn't be nearly this lopsided. I think a large part of it is that everyone's seen how bad the balrog looks in the movie, and just votes for it. I would have to think a lot longer to come to any sort of conclusion, but I imagine it would still depend on which particular dragon or balrog was fighting.