woah, one second...
Glaurung would, it seems, have a great defensive advantage due to his massive size, and
Ancalagon would not have too much of an advantage in the air.
Ancalagon would have greater mobility, but in order to fight the grounded Glaurung, would have to forgo the wings anyway.
Also, what is the basis of the argument that Galurung wouldn't be able to use spells on Ancalagon? My opinion is that the spells would be based on the strength of Glaurung's mind as opposed to that of Ancalagon, not their status as
dragons.
If the fight came down to a simple, head-on collision of mind and body, it seems that the first
dragon would have an advantage.
Of course, on any given day, any dragon can defeat any other dragon, it would depend on what tactics Ancalagon would use, and the terrain, and the situation, and teh emotional states of both dragons, etc...
sorry if I took this a little too far,