Become ethereal--close the distance--dagger in the back.

Voice wins.

I think they'd both have the power to obliterate each other many times over, so it'd come down to speed and who picks the right opening move to counter the other's. The thuumer could start with a quick fus to interrupt the mage's casting then finish them off with a stronger one, or they could start charging up ULTIMATE SHOUT FORCE right off the bat.

The mage might try some kind of ward, hoping to block a faster, weaker shout, or go on the offensive immediately to beat the shouter before they can finish a full one. Whoever wins it'd probably be the kind of fight that'd be over in two seconds and leave a crater.

Neither wins, they are both so focused on mastering the voice or magick they forget to learn battle tactics. So they both go on the offense and kill each other.

One good question.

Is this skyrim magic only?

If it is, the Master of the Voice wins no contest.

Else it's a tie.

Slow Time Shout.
Stab in face.

That is all.

Why do you guys keep suggesting the dude attacking while he's in Etherial form? You know you can't attack at all while you're in that form right?

First shot wins - either side. Like many battles.

The silence spell, although it only affects magic in game, actually does prevent sound from travelling in lore. So if a person were to use a shout, but be under the effect of a silence spell, then they would be powerless. (This is of course assuming that shouts work by travelling sound)

http://uesp.net/wiki/Lore:Silence

actually, you're wrong there. you can attack, but the shout will wear off immediately. i've done it a million times.