As a pure track car without making any mods the 996GT3 is a better choice. But the factory stock 996GT3 is NOT a compliant track car, without mods. I don't think you can go wrong either way. Just don't think for a minute that either car will perform as a super fast, compliant track car after your first season as a novice. For season 2, if you want a superfast track car you need these mods:

BOTH 996GT3 and CaymanS:
More negative Camber, monoball bushings, racing seats, harnesses, roll bars, track wheels and tires, lightweight exhaust, lightweight Flywheel.

CaymanS needs these additional mods:
Coilovers (Bilstein PSS9 or better), Tarett Camber Plates, 997GT3 lower Control Arms; adjustable swaybars; under drive pulley, Mantis sump extension; MotorSport oil separator; Power Steering cooler and duct.

There is a certain satisfaction for being noticed as a Cayman out performing GT3's, GT2's and other cars with 50% more horsepower.
Full disclosure: I am looking for a 996GT3 and I am on the waiting list for a 997GT3. Not sure which way I will go.

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CaymanS Track Videos
2004 996GT3 / 2007 CaymanS / 2008 SLK350 / 2007 Cayman S Track Car (Modified with parts from: 997GT3, 996GT3, 997GT3RS, Porsche Motorsport, Tarrett, Mantis, Arlan, AP Racing, Moton, Eibach, Tubi, Aasco, Sharkwerks, Quaife, TC Design, Heigo, Fikse, Softronic, TeamTech, Traqmate, SVtek, ChaseCam, Pagid, Rennline, Hooiser, Motul)
Complete Mod List and Setup Info