I think it's honestly very difficult to tell who's stronger and who he would have more of a cartonish G5 fight against, thus it's very difficult to predict the match ups.
Against Kizaru he could do some cartonish stuff against, by, shooting back his beams at him with a baseball bat or creating a mirror in order to redirect Kizaru's yata no kagami and such.
On the other hand, if Saturn is some kind of Bull Zoan, he could pull out some Disney's Hercules vs Nessus (the centaur river guardian) fight with it. Oda loves that kinda stuff and he's a huge Disney fan.
I'm still leaning towards Kizaru tho:
1) He seems to be his bodyguard, indicating > strength portrayal.
2) Saturn's abilities might resemble Kaido's too much: he legit might be a Centaur swinging around a CoC infused stick (obv first impression)
3) if point 2 is true, it might be more fun to see how Zoro deals with a light version of Kaido (simply in terms of powerlevel and abilities), while seeing how Luffy deals with the Pika no mi's awakening. The other way around it might be too repetitive.