Too vague in my opinion. So, without any extra details I assume neither has knowledge and they are both fighting normally. In this case, Onoki probably wins simply because he has shown the preference for using his stronger jutsus from the get-go. Jiraiya is more peaceful and doesn't exactly just start fights in his incomplete sage mode. Nor does he summon Ma and Pa randomly.
Basically, Onoki wins early because Jiraiya will be hit off guard with dust release. If they get to the point of going all out (sage mode, ma and pa, etc) then Jiraiya's odds go up significantly and the fight actually probably favors him at that point. But like I said, it probably will never get that far.