I'm leaning more towards SM Jiraiya though Muu is rather reflexive, managing to evade KCM Naruto's blindside attack mid-charge (for Jinton) and managing to react to Base Ei appearing right in his face despite being weakened (due to his body being split in 2) implies that he has a fair shot in challenging SM Jiraiya. Additionally, this is pure speculation on my part, but he could very well have access to Onoki's Weighted Technique that increases the mass of an object(s), so that in itself can be rather useful in dispatching summons and obliterating them with Jinton.
I'd still give it to SM Jiraiya because he has a viable counter to Mu's Camoflauge technique even though the fight itself won't be easy. Mu has everything that allows him to stay outside of Jiraiya's range, but Jinton itself is a rather taxing technique.