a properly made X pipe will offer a bigger power band because it increases flow and offers better equalization in the critical low and mid range power band. If the system is not restrictive, the X or H pipe can not increase flow over 100%. On a Z with a 2.5" system, the X pipe will shine. A 3 inch system with minimal bends on a Z is large enough, that as long as you have something in the system to equalize pressure, your starting to split hairs. X pipes generally offer a better tone because they blend the sides of the engine more efficiently. H-pipes tend to have more of a spit sound at high load because your hearing the separate sides of the engine more.