Itachi wins.
Jiraiya is lucky Itachi left with Kisame. Even Kisame asked "Why is a retreat necessary for you?" (The story line is what prevented that battle)
Because if he wanted to, he could've just easily used:
Amaterasu: Jiraiya was just standing there, and could've been Amaterasu'ed easily, and theres no way he can dodge it. Jiraiya isn't Raikage lightning armor V2, speed, So He wont be dodging it. Not even Sage mode is faster than the 4th Raikages lightning armor V2.
Tsukiyomi: Itachi also could've used that too if he wanted because Jiraiya was staring straight at Itachi.
Genjutsu: Itachi could've easily used one of his Genjutsu's to immobilize JIraiya. I know Jiraiya can escape a regular basic Genjutsu, but Itachi wouldn't let him, just like how he did Naruto. And all Itachi has to do is use a genjutsu, that isn't recognisable.
For example like, not changing the surroundings would leave Jiraiya clueless, and unaware that he's even in a Genjutsu. Itachi did this to Deidara too.
Sorry but Itachi is a little too much for Jiraiya, despite what he said, saying he's a stalemate. Which isn't true. It's stupid when people think Itachi will lose JUST because he said he's a stalemate. Compare their abilities and intelligence and strengths THEN decide. Don't just base an battle on "What someone said". Itachi only doubted himself because Jiraiya's title is "Legendary Sanin" which means NOTHING AT ALL, since Itachi already owned a "Legendary Sanin" (Orochimaru), in 10 seconds flat.
So you can't base an battle, between them just because Itachi doubted himself.
Pain also said" If he had known my secret, we might not had of won". That dosen't garuntee an automatic win. It's just an estimate or hypothesis basically.