Well this is a collision slated to happen around spring 2014.

From my reading Didi is already better defensively than Zack. Here's a scouting report..

Scouting Report
Didi Gregorius is arguable the best athlete in the entire Reds farm system. He has plus speed and athleticism that is only rivaled by Billy Hamilton. Defensively, he has above-average range at shortstop to go along with good hands and an outstanding throwing arm.

Experts in two years the Reds have a decision to make it looks like