To me they are completely different. The Zen is one of the most controllable balls that I've ever thrown. If you drill it pin up over the bridge, you'll get a gradual but angular motion on the back. If you drill it pin right of ring finger and stacked, you'll get a continuous arcing motion on the back end. It carries very well.
The Phase III is a ball that doesn't rev up in the front of the lane. It is very clean through the fronts and saves its motion all for the backend. Kind of long and skid snappish to me. It's not a bad reaction at all. You just have to play with it and get used to seeing it.
