The S2k is the best for your money. they are 30k+ where as the s2k is usually around 20k+. Also the z4 doesnt have the traction control the s2k does. The S2k will haul some ass esspically when you do a few mods like the intake or the ram air. The s2k even as the Vtech and once it kicks in (around 3.5-4k rpms) it will leave the z4 in the dust!!!!