I'm considering the 2010 G37x sedan. Looking at packages it looks like the Tech Package, which includes the Rain Sensing wipers, mandates the purchase of either sport (not interested) or the Performance Tire package.

What is the advantage/disadvantage of getting 18" vs the standard 17"?

 

I believe the 18" are Dunlop Sport Maxx's and the 17's are Goodyear RS-A's. The Dunlop is a much higher rated tire(check TireRack). Typically 18's handle better but you'll feel road imperfections more. 17's may be somewhat better in snow etc. I suggest you drive both at dealer. I did and ordered the 2010 G37xS with the 18's.