The Vibram IdroGrip sole on the Z/2 Pro is a vast improvement in terms of wet/dry traction. I just finished a several day fishing/whitewater rafting trip and think that the traction these shoes provide probably kept me from falling a few times. Scrambling over smooth, riverside boulders (dry or wet) was easy, and traction was much better than with my regular Z/2's or Keen H2's. In-river traction (on mossy rocks) was as good as it gets for a non-studded shoe. Chacos provide good arch support, and the strap allows you to have the sandals as tight or loose as is comfortable or necessary. Also, I think that the toe loop of the Z/2 provides quite a bit more stability than you have with the Z/1.
In short, this is an excellent shoe if you are in and out of water, or do hiking that involves some scrambling (where traction is very important). However, it is a marking sole, and can wear down fairly quickly. So if you are just wanting a sandal for everyday use (or casual hikes), the Pro sole probably isn't necessary.