NelsonF:
--Wow, I've never heard of someone with this experience after driving various 911's. With all due respect, If you really prefer the 4wd C4S over a GT3 after driving on circuit, then I have to think that the GT3 had a poor alignment set-up or some other out of spec malady or you were not really pushing it enough to note the high speed handling difference. Perhaps your sports car experience is different than most or you really prefer the feel of 4wd?
The Silverstone Porsche center is not just a handling track, it's also got water slopes, skid pans etc. The C4S 911 really especially shines in these other areas way beyond the GT3 principally because of it's 4wd and it's PSM. Yes the GT3 is a lot lighter so easier to control when the back steps out, but in terms of a road car there is no contest. The only place the GT3 was faster was on the handling track. HOWEVER, this is a very tight handling track designed to mimic a twist UK back road with rises and falls - where you can't get anywhere near 100mph at any point. So yes I am sure the difference would have been more noted on a bigger track where you can carry a lot more speed through corners.
I am also sure the GT3 (with full race spec) was set up properly as my instructor for the day was the Porsche Cup winner in 2003 and he drives the cars every day and seriously knows his stuff. Turn in was way sharper than a Carreras. This is easy to achieve though by taking any 997 down to a suspension tuner who knows their stuff (*see any of my old suspension tuning threads). The best thing for me was the bucket seats - they make you forget you are actually taking a corner at all!
With regards to my personal preference of 4wd over 2wd in a 911 I have to say that a lot of it is down to steering weight. When accelerating I prefer the front nose of the car not to go light. This is a major 'feature' of the 911 (with it's weight primarily over the rear axel) so anything to balance this out, like having 4wd components weighing down over the front axle gives you much more steering feel when accelerating. It's just much more driver confidence inspiring.
Again though, I am a 95% road, 5% track guy - so my main focus is public road driving. The GT3 is supposed to bridge that road/track gap - with the lack of power though (compared to what I am use to) and on the track I took it on, I sadly found it a serious disappointment to what I was expecting. The difference between the GT3 and the C4S would only be worthwhile should you be a 50% (or less) road and 50% (or more) track guy.
PS: I asked what 911 the instructor would have should he be given one, and the 2003 Porsche Cup winner said "a Turbo" 
I couldn't believe it, I was expecting him to say a GT3RS or a GT2. I suppose when he drives on a track though anything less than a RSR is just not good enough, so something tailored specifically to public roads is ideal.
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but if its any indication my 2.5 yr old daughter preffers to ride in the back of "daddy's car" than "mom's" when asked
)
(as the carrera gt is totally out of budget) 
