If it does come down to one on one, I think the prophet could win. Depends if Ikoral gets his Chosen or not, I guess.
I agree on that. If Ikoral gets his Chosen, he wins for sure. If not, the Prophet has a chance.
(I think he actually would take his Chosen with him. Judging from the confrontation of Thanaton and the SI, the Kaggath has no rule against allies for this. Thanaton calls on his Cyborgs, the SI on his/her companion.)
So, all in all seems that Ikoral has the edge. Vodal could win if his followers can carry on a successful guerilla war or if Ikoral decides to engage the Prophet one-on-one. But they only have a believable chance of winning, they are not clearly superior in these areas.
Right now, Ikoral comes out on top.