I have been shooting with a Z for the past 8 months. I bought an I-3 3 weeks ago and have been using it since. IMO, they play identically deflection wise. Feel wise they were also very similar to me. The tip diameters are the same, but the I-3 has a much stronger taper. It looks completely straight from the tip to the joint. The Z has a tiny bit of a curve to it, so it is thinner in the bridge region. I'd say its a toss up between the two. One advantage the I-3 has is you can get it in any lenght and in any joint, where as teh Z only comes in 29 inches and Unilock.
I'll post a more lengthy comparison of them maybe next week, including pictures, taper measurments, and quality observations.