The standard 996 and GT3 have nothing in common as far as engine and transmission. The standard 996's "integrated dry sump" is really a marketing gimmick designed to obscure the fact the engine is really a wet sump engine.
Be assured the GT3 is a true dry sump engine based on the very first 911 engine's crankcase. The same crankcase is at the heart of every racing 911 engine ever made and every Porsche racing engine ever derived from the 911. The current GT3 engine is in the same series as the GT2 and Turbo, and the Le Mans winning GT1 engine. It is the real thing, Titanium rods, replaceable Nikasil cylinders and all....the standard 996 is junk by comparison for racing use.
The gearbox in the GT3 is derived from the G50 gearbox of the air-cooled cars and has a competition heritage as well. You can get all kinds of LSD's, gears and oil pumps etc for this gearbox. Same comparison to standard 996 as above.
There are a wealth of racing parts for the GT3 from the GT3 RS and RSR and Carrera Cup cars for the suspension and engine available from Porsche Motorsport
If you want a reliable car to track for less money to buy and operate get a GT3 IMHO.... and there would be no questio whether you could run PCA events