1. 997 Carrera S with 20 mm chassis is slightly faster on the track than a 996 GT3 Mk1 and also better controllable, especially by not so experienced drivers.
2. 997 Carrera S with PASM is as fast as the GT3 MK1 on the track, maybe slightly slowlier on certain tracks like the Nordschleife for example.
3. The 997 Carrera S stands no chance against the GT3 Mk2 but in the hand of an experienced driver, it can outrun most GT3 Mk2 which are driven by not so experienced drivers.
All my assumptions/experiences of course with chrono sport activated and PSM turned off.
Another "fact": the GT3 Mk2 is around two seconds faster from 0-125 mph than the 997 Carrera S, top speed is also much higher.
I had the choice to go for a GT3 Mk2 or a 997 Carrera S for the same money. I did choose the 997 Carrera S because my track days are almost over and I enjoy to have a safety cushion (PSM) in my car. I also like the new look of the 997 Carrera S, the low insurance cost (over here in Germany), low maintenance cost, etc. and of course the two seats in the rear for the two little kids I have.
If you're not very often on the track or if you rarely have the chance to drive the car very very fast (speed), the GT3 Mk2 might be the wrong car. But of course it is also a matter of personal preferences and a little bit some sort of EGO thing. Driving a "GT3" is always something special, while driving a Carrera may not be THAT special. Of course this is only important if you meet a lot with Porsche addicts, most people on the street don't have a clue about the GT3, body kits, standard equipped cars, etc.
If you want to go for a GT3, DO NOT buy the Mk1 version.
Unless it is heavily modified, it can't outperform the 997 Carrera S.