Hmm...well, just off the top of my head (my copy of the book is not handy at the moment, but I just finished a re-read recently), I don't believe it is ever mentioned that
Glaurung was any stronger than
balrogs. However, while he was a servant of
Morgoth, it's not clear that Glaurung was particularly beholden to him in terms of strictly acting on Morgoth's orders
only. As far as I can recall from my reading, it seemed to me like it was more that Glaurung was simply "let loose" upon the land to wreak whatever havok he wished, rather than follow a particular plan. In his wanderings, then, perhaps it struck his fancy to go look for
Nargothrond and give it a nice sacking.
On the other hand, it is clear that Morgoth wanted to absolutely torture
Hurin by making him watch his children suffer. To that end, perhaps Glaurung was more cunning and wily than the Balrogs, what with his psychological games and all when taunting
Turin at Nargothrond. The balrogs, on the other hand, struck me as simply powerful
weapons / warriors, but not of much purpose beyond that.