It'd be a close match...BUT...Jiraiya would probably come out on top. Remember, he's one of the Sannin and has about 30 years of experience over Itachi. Let's explore, shall we?
Jiraiya comes across as weak only because of sheer circumstance; he was drugged by Tsunade during the fight against Orochimaru, he was up against two Akatsuki members (one of them being Itachi) while having to protect Naruto - who was their target - and Sasuke at the same time and still forced them to flee rather than fight, and he lost to Pain only because he didn't know the secret of Pain's techniques. Pain even said himself that Jiraiya would have won if he'd known. Also, the reason why he couldn't stop Orochimaru from leaving the village years ago was because he was younger with less experience, and because he was trying to bring Orochimaru back alive, so he couldn't use his full strength.
Itachi is a master of Genjutsu, and would likely try to trap Jiraiya in an illusion. But, wasn't Jiraiya the one that taught Naruto how to break Genjutsu? Who then used it to break an illusion created by Itachi? True, it was a clone with 30% power, but Naruto was only a genin and still managed to break out using the technique Jiraiya taught him.
Furthermore, Jiraiya can summon the Elder Toads, who both specialise in Genjutsu and can be used against Itachi. The genjutsu they used could kill anyone who heard the sound of their warbling, which was very loud. Jiraiya could use them to back him up, as well as all the other toads.
Itachi is a legendary ninja with significant skill, but Jiraiya is as well. He's infamous. Plus, he has the Rasengan and his oil techniques. A testament to Jiraiya's skill is this; he's had five students; Naruto, Minato, Konan, Yahiko, and Nagato. All of them have grown up to be incredible ninja with massive power, particularly Naruto, Minato, and Nagato (who is quite possibly the strongest Shinobi in history (barring Hashirama, Madara, and the SO6P)).
So, all in all, it'd be a very close match, but I reckon if they were both going all out, Jiraiya would probably win.