Ok, I'm coming to this a little late, but here goes.
First of all, I don't think that the dragons are infused/indwelled/inhabited/whatever by Maiar. The ones that I've read about seem to be more like (big and strong and fire-breathing, but ) humans. Actually, I know some people who act like dragons (meaning Smaug and Glaurung), at least in personality. Dragons, in Tolkien anyway, are greedy, arrogant, and generally all-around .
On Balrogs, well, they just seem to exist for one purpose. Destruction. Of Elves, of dwarves, of men, of anything. They are more like demons than dragons are.
Also, I do agree with someone-or-other (sorry that I can't remember your name) that the dragons are very smart (and ) animal sort of things. Balrogs are more like just plain demons. They are "Maiar in a purer form." (though I still don't think dragons are Maiar at all)
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