for one they're cheaper and tires will be cheaper also. They are going to be lighter and like mentioned above can be rotated assuming same size tire is used(recommend a 275/40). A 17 inch wheel/tire will also give you more protection from bending a wheel. I love the FN01rc's that I picked up from SPL. With a 275/40 hoosier on there it weighed in at a mere 42lbs. If money is no object the 18's will work and allow you to run a slightly wider tire in the rear which might be advantageous if you're putting down big power.
For all around function the 17x9's work great for me.