S2000, because it out performs the crap out of an IS screw looking pimp, i go faster then you and handle better, with the top down period. The aftermarket is just as good if not better for the s2000. I've driven both while the IS has some high points i would only buy it if i wanted a sedan, and for the money you would find me in an EVO long before an IS. The only way the IS is fast is with ALOT of money ie: turbo kit from 4K up to a 3s-Ge swap

Road & Track 2003
S2000 0-60: 5.5 0-100: 14.2 1/4: 14.1 Skid: 0.90
IS300 0-60: 7.0 0-100: 18.9 1/4: 15.4 Skid: 0.83