Ozai vs. Iroh

Who would win in a battle to the death?
Location: Where Zuko and Azula had there final Agni Kai
During Sozins Comet.

Ozai would win. Only barring his total ruthlessness and willingness to kill anyone to accomplish his goals. Even if it's a fight to the death, Iroh would definitely hesitate at killing his only brother.

While I actually think Iroh is the most skilled of the two, I don't think Iroh has what it takes to kill Ozai. Ozai on the other hand...