It's hard to say but I think the voice is more powerful. You can destroy somebody with a single utterance, and if you're powerful enough you can make the world shake. Normal magic could never make a dragon understand mortality...
This is true, however you'll note that Agnir doesn't totally blow you off the map when he opens his mouth, and hes their leader. My assumption is that Dragons don't have that horrible side effect, and are able to control it, as opposed to the Greybeards who take a really, really, long time to get to that point.
On righteous Thu'um master vs. righteous magic master. To be completely honest, I don't think that there's any chance that either of them will wind up getting a upper hand on one another. It really depends on what kind of skills each opponent utilizes, and I believe the victor would have to go outside the magical box to claim victory. Epic battle tales usually go down like that, as opposed to a couple of dumbasses punching each other in the middle of a cornfield until they get tired.