I have a high adjustable rib on my Zoli Z-Sport. I like it for the most part, but found that I basically just set it in the 55/45 position and never change it, since I also most only shoot sporting. If you are shooting just sporting, I wouldn't recommend the high rib. It definitely gives you a more comfortable head position if you have a long neck. With some target presentations the high rib was a disadvantage for me (ie dropping targets). If you are going back and forth between sporting and trap the adjustable high rib can be an advantage.
When I bought my Kolar, I went through the same debate as you deciding on which rib, but I knew I didn't want the high adjustable rib. I ended up getting the Ramp Tapered Rib, which is Kolar's mid-rib. After talking with Robert Paxton, he told me that there was only a 3/16" difference between the Ramp Taper and the Flat rib, but the Ramp Taper gives you just a slightly more heads-up position. I really like this rib configuration, and dropping targets are much easier for me with it. It is a little harder for me to keep my head on the gun having gotten used to the high rib, but I'm getting better and better with it. I don't think you can go wrong with any of the rib configurations, but definitely look at the Ramp Tapered rib.