kishi hadnt yet planned how strong to make itachi that's all, there's a big discrepancy between part one and two.
in shippuden, kishi made itachi and jiraiya equals. equal score in databook, same manner and time of death, similar life and feats. itachi had just had a lot more screentime because he's more important to the story, and got shown as a free edo tensei. jiraiya's feats are largely off panel including over a hundred s-rank missions. itachi has the edge in genjutsu and ms, jiraiya has the edge in bunshin and sm, itachi is smarter, jiraiya is more versatile in tactics and strategy. it could go either way.