[*13*]

Traction is mostly covered, above. Looks aside, There is a noticeable difference in ride quality between the two. My 15" feel nice & plush. Good tires are the key to the level of traction attached to any size. My 17" grip nicely but transmit every crack in the road into my @$$.
If I had to rely on one set, it'd be the 15". Of course, I don't track the car. & if I did, It wouldn't be serious enough to necessitate 17" wheels. So it's situationally dependent, right? You can't make a wrong choice. DO what your gut tells ya