[mikeinatlanta]

Reasons for the 15" wheel
Lighter
Authentic look
Lower CG with low profile tires
Better leverage ratio for the brakes

Reasons for 17" wheel

Tire selection
Can fit bigger brakes
Assuming the same diameter, the lower profile reduces the effect of the tire "air spring" and its uncontrolled lack of rebound damping. The greater the ratio of unsprung to sprung weight on the car and the more flexible the chassis, the more pronounced this effect is. Lower profile rubber transmits the suspension movement more effectively into the springs and shocks, allowing better control of suspension movement and improving the effectiveness of spring and damping adjustments.

If you can find appropriate rubber and they fit over your brakes, low profile 15" would be of the biggest benefit to a street car that normally runs taller rubber. Problem is, you won't find appropriate rubber.