Well,
Dragons have SOME sort of spirit in them.....
"Then
Glaurung spoke BY THE FELL SPIRIT IN HIM".... Just happened to read that a few hours ago.
As far as
Balrog vs
Dragon, so many variables, but I think that size & power would rule there.
Every dragon I've seen a detailed report of death on, save the utterly ridiculous
Ancalagon the Black vs
Earendil in the boat in the sky story, was a matter of 'a shot in the dark' to a soft underbelly, not overpowering. And, face, stealth is not a Balrog's strongpoint.....
Gandalf didn't fear the Nazgul, whilst he seemed quite worried when Durin's Bane decided to appear during his trek through Moria.
Denethor: "Is it possible you withdrew (from the
Witch-King/
Black Captain) because you had faced a foe by whom you were overmatched?" (paraphrased)
Gandalf: "How Dare you! I'm Don't fear him!"
No.....
Gandalf: "Could Be, Our test has not yet come..." (paraphrase again)
BTW, Gandalf never said he was worried about the Balrog.....
"I have never felt such a challenge. The counter spell was terrible."
"A balrog. Now I understand... And I am already weary."
"A Balrog. Now I understand."
Is it possible Gandalf had dealt with a few
Balrogs as part of The
Host Of The West at either
Utumno or
Angband in the past?
Good possibility.......
Oh, to answer the original question, no doubt, similarities, but Balrogs are by far greater than Nazgul.
Here's another '
food for thought'......
What if Durin's Bane had discovered
The Ring in Moria?
Was he
Dark Lord material?
I'd say yes, if just by default in TA
Middle Earth.