Where is the option for driver mod?

I voted suspension, because the corners is what I enjoy... but if you are just tooling around on the street, might as well keep it comfy and go with power. A stock G35 should already be excellent in both power and handling... but since you are itching to burn some money on it, get whichever you would enjoy the most.
Personally, I enjoy driving my cars around a racetrack, but you may prefer to spend $$ on material goods rather than experiences.