I think Zepeda has bigger chance due to his punch and speed. While his chin looked little wobly in his last fight I think he will catch Ramirez who looks greener in my opinion.
Ramirez has faced some good opposition in Orozco, Reed and Imam but are they on par with Zepeda? Honestly, I don't think so. I think it will be close fight untill someone lands big punch.
Zepeda have more fights and have more power, plus he have good workrate but that being said, Zepeda has more chance against Ramirez who may pak weaker chin. Battle of chins if you will.
His gas tank really worried me in his match against Carlos Diaz Ramirez, which is the real reason he went down, but he recovered brilliantly, and stopped him the next round. This will be perhaps the toughest fight of Ramirez's career and is a genuine toss up.