I can vouch for the TPIS shocks, and highly recommend them...simple, no nonsense shocks that give a subtle, controlled and "flat" ride. My experience with KYBs on a C3 was that they had a sharp rebound that unsettled the car on certain types of road surfaces. However, many people are pleased by their performance on C4s, so this may be simply an issue with the valving for C3s. I'd specifically ask owners of C4s with KYBs whether they notice this characteristic on their cars. I have had good experience with Konis, but they are very difficult to adjust. Somewhat ironically, the best shock on my old C3 was the Monroe Sensatrac--very subtle and controlled. They really calmed and gave control to what could be a nervous car, at times. They make them for some (but not all) C4s. I have been disappointed in Bilsteins, both as to the product and their support services, and cannot recommend them. Others, of course, feel differently. Bottom line: I'd go with the TPIS--at least if you don't like them they are relatively inexpensive mistake.
Chris Kennedy
Houston, Texas