All in what you want the car to do, ride, handle... And what looks "right/good" to you. You'll be the one driving and looking at it the most so again it's all up to you.
I love the 17's I have from Compomotive. The 315/35/17 BFGOODRICH Comp TA/KD's in the rear grab like nobody's business.
This is what looked "right" to me...
http://www.clubcobra.com/photopost/s...t=1&thecat=998
Isn't the agony of the decisions GREAT!!!