I will be coming to this decision soon also.
I have a couple points I want to air, and possibly clarify.
1. The 15" or 17" wheel will have different aspect ratio tires, won't they?
That should mean that the circumference of the tread should be about the same.
Thus, the shorter, stiffer side walls of the tires on the 17" wheel give a harsher ride where as the tires on the 15" wheel has more side wall rubber to absorb some of the road imperfections and improve the ride.
2. The traction from a tire is a product of weight and coefficent of friction.
Given that two tires, one a 15" and one a 17" on the same road surface and are made of the same compound, the handling charactistics (other than the above side wall stiffness) will be the same. The width (area, or contact patch) has no effect on the tire's ability to grip the pavement.
I understand that the shorter side wall will have less roll over and therefore possibly more contact area, but area is not a factor in calculating the fricton force.
I do, however, accept that the 17" wheel can support larger brakes.
This makes for a brake that has more mass and therefore will take longer to overheat and fade under similar driving conditions.
The dissipation of heat is the primary energy transfer in braking. The tires need to handle the heat as well as the brake pads and discs.
The larger wheel will also pass more air to aid in brake cooling and resist fade better.
For a street car it comes down to the choices in looks and cost.
IMHO the performance will be essetially the same.