If the s15 is anything like the 13 (chassis wise) it will handle like a go kart while the 'line is less forgiving. This means thats its a safer car if u lose the back end but it will make u wanna do more stupid things in it aswell. Cmon we all do stupid things at one time or another.
S15= short sprints, high revving, all up a really "go hard" car. Easy to up the p[ower substantailly with just an exhaust.
r34= long country drives with power to get up and go on the freeway and in the city. Plus ure chances of fitting someone in the back are a bit higher but probably still not practical. U should buy a HSV:D