dragons vs. balrogs, eh? well, in the tales of dragons, they are usually more free agents, sort of. they seem to kind of do as the please, at least more so than balrogs. also, maybe that is why stealth doesn't work on balrogs: they are always holed up on a tower or on the field in a huge battle, whereas dragons make themselves comfy on a pile of someone's gold in a deep cavern somewhere by themselves. it is interesting to note, thought, that dragons very precise senses of hearing and smell (Bilbo and smaug episode), so they seem to have some defenses against stealth.
other than that, there is the obvious difference in flight capabilities.
