I've always been an Olivares man, but realistically, since I do rate them on my personal all-time bantam list at numbers 1 & 2 respectively, and they are two brilliant bangers, anything could happen.
I honestly don't know. People are usually higher on Olivares but I'm not sure what he did that Zarate couldn't do.
He could box better for one; get on his toes, jab, circle that sort of stuff. Knockout streaks aside they were very different. Zarate stalked guys, wore them, and then blasted them out whereas Olivares would bounce around and unload hooks with both hands. Even their body punches were different; Olivares had the classic Mexican left hook to the body while Zarate favored a left uppercut. Zarate was a huge guy whose style was not conducive to moving up while Ruben moved up and was championship material up a couple of weights. Plus Olivares beat much better opponents. I'd take Ruben but Zarate was very physical and might be able to win.
I believe Olivares would take it, just a more complete skill set. I also believe Eder Jofre outboxes both.
With Olivares being able to carry his weight better he might be able to outbox Zarate and gut it out down the stretch. I am leaning towards Ruben now.