Iroh is older now, he's not one who wants to fight now either. Though Azula is very evil and probably would fight iroh. I voted iroh because he has more experience and can avoid azula's attacks eventually tiring her and finish her off.
Hes not a spirit and doesnt have infinite lives. He can only live because he's in the spirit world. Also, ignoring all morals, Iroh is pretty slow for an all powerful firebender and Azula does have blue fire / lightning