[Farael]

Well, I think it's a matter of three possible scenarios

a) Morgoth and Sauron going "mano a mano" in a one-to-one fight. Sauron may have had the edge as Morgoth had spread much of his power around.

b) Sauron revolting against Morgoth, gathering an army and going after his former Master.

c) Sauron gaining the loyalty of Morgoth's strongest lieutenants and captains and overthrowing him without a battle.

Of the three, A) is the only one that would have Sauron as a victor, but at the same time it would never happen as I'm sure that Morgoth knew of his own limitations and at the same time, there are no indications that Sauron was unhappy with his role (as far as I am aware of, and I reckon I don't know as much as other)

Option C) is all but impossible. Where has Morgoth's power gone? into "creating" (I know, he can't actually create, but he did modify existing things to serve his purposes) lieutenants that would not only be mighty but also respond to him. It is possible that some of this power Morgoth spread went to assuring the loyalty of his men.

Option B) has the same shortcomings that option C. While Sauron may be able to gather an army of orcs to march against Morgoth, it is very likely that Morgoth would have most if not all of his Balrogs, Dragons and such creatures by his side. No matter how great an army of Orcs Sauron may be able to gather, they would be no match for Morgoth's forces.

Sure, Sauron may (and note it says "may") have had the edge on a movie-style one on one confrontation, but Morgoth did not spread his powers around by accident. In a way, it's like Sauron's ring. Sauron "spent" some of his inherent powers on making the ring but at the same time he became stronger while he had his ring. Morgoth spent some of his powers on creating his vast armies and well, as long as he had his armies, balrogs, dragons and other such demons he was an overwhelming force to be reckoned with.