One eye contact with Itachi can kill you, one glance of him can, too. He's the best at Genjutsu and even possesses Shisuis eye, calling it a part of his power, while Jiraiya however is weak at Genjutsu and gives Naruto the advice to run away if he's about to face a Genjutsu user in a 1on1. He's much faster than Jiraiya in both speed and handseals, so fast that not even the Sharingan can keep up with him properly. He is also much more intelligent, according to Kabuto able to read his opponents like an open book.
Kabuto needed to close his eyes, in order not to fall for any of the Uchihas Genjutsu, above, the Uchiha mustn't kill him. So what makes Jiraiya better than Itachi, if he's slower, less intelligent and can be one shotted with one eye contact and glance of Itachi? I dunno.