i dont know alot about sti's, but a chip for an s4 gets more gain than a chip for an sti, so a chipped sti and a chipped s4 would probably be pretty equal, ive raced a chipped evo and beat him by quite a bit. but ive got dp's diverter valves, lightweight flywheel, and some other stuff.
i love sti's though, i cant decide if i want to sell my s4 for one or not, i know they both do just as well on snow and dirt and all that, but the sti is easier to mod and i wouldnt mind trashing one. but then s4's are so damn nice, and they are such sleepers.