which one would provide better traction? i have seen most guys run the 15's , why not the 17's

 

You have more profile, this is way 225/50/15 gives better traction

 

To translate that into English, the taller the sidewall on the tire, the harder you can hit it without getting wheelspin, all other things being equal. A drag radial, even when aired down farther than it should be, won't "wrinkle" like a bias-ply drag slick, but there's still enough flexibility in the sidewall to act like a spring. The more sidewall height, the more spring action. When you launch the car, instead of instantly transferring all the torque to the tread face, a tall sidewall stores some of that energy, and then gradually releases it as the car starts to move.