Both tires are excellent and get good reviews. Michelin Pilot Super Sports are also an excellent choice for both street and track use and will be more friendly for cold or wet weather conditions.
Rival's currently are only available from Tire Rack, but I here that is going to change as of the first of the year. Note: Rival's need a full heat cycle to be effective on the track. Falkens are more expensive and can be purchase online and also at Discount Tire.
Also, Dunlop has some excellent new tires and I like the Bridgestone SO4 tire as well.
I would recommend a 275/35-18 front size as it is a little taller than the 265 tire.
Good Luck!
