Re: C2S vs C4S
A never ending saga - just search it and you'll find this debate goes on an on into the ages. This thread is particularly friendly to the C2, while most others are more balanced.
A quick word about the understeer mentioned here and elsewhere: 911's naturally understeer. The engine weight over the rear axle makes it very difficult to lose traction with the rear wheels so that under most on the edge conditions, the front wheels will lose traction first, hence, understeer. The car already has a 40/60 front/rear weight bias. Now floor it in a turn, and shift even more weight off the front wheels, and voila, you go straight into the wall, or preferably, onto the grass. Thankfully, the natural tendancy in this situation is to lift off the throttle, which transfers weight onto the front wheels which adds to their grip, allowing the driver to often regain control before reaching the wall/grass.
Of course, the 911 can be made to oversteer very easily: abruptly lift off of the throttle in a turn, and weight is transferred off of the rear wheels. That reduction in rear wheel grip (and extra front wheel traction), combined with a large polar moment of inertia (created by the weight hanging over the rear end of the car), and the back end will swing out. This is a counterintuitive situation, for to regain control of the car, one must get back on the throttle, which goes against natural instincts. Another way is to "power oversteer": put enough torque on the rear wheels that they overcome their frictional coefficient and spin. The latter is not easy to do with a 911, and the throttle must be applied gently (squeeze, smooth), or else understeer (as described above) occurs first.
All C4's, with the possible exception of the 997, understeer much more than their C2 counterparts, partly due to something called torque steer. This is also counterintuitive, but when you put torque on the front wheels, part of their traction is used in applying that torque, and part to changing the direction of the car. Because the tractionis shared between these 2 tasks, some of the traction used to change the car's direction is lost (to the other task). This is a gross oversimplification, but it is true, and those of you who have driven front wheel drive cars know it implicitly: they understeer like pigs.
The 997 was engineered to dial out the understeer of previous C4's, and by most accounts, those efforts have been successful. I plan to test drive one this summer to see for myself, as I remain highly skeptical.
Am I considering a C4? Absolutely not. The 911 already has incredible amounts of traction because of it's rear engine design. That is it's main design advantage. Thus, to me, there is no need for a C4, unless one plans to drive in low traction conditions frequently, or one is harnessing 500 hp. I want my 911 to be as close to a real sportscar, and as different from any sedan or other driving experience as public roads will allow, so that means RWD. It's got incredible steering feel, and no problems with traction. On rainy or cold days (when tires grip less), one doesn't need AWD, but rather, a small dose of common sense. The C4 costs more, and to me, it's only real advantage is those sexy hips, which I love, but just can't justify (though I would love a 997 GT3 RS, since it's RWD AND has the big hips, not to mention many other fantastic goodies, hehe)
If I was considering a C4, I'd wait, since the electronic AWD unit on the 997TT is likely to be available on the normally aspirated 911 C4's in the near future. The current viscous coupling unit is old fashioned, and so far, reviews of the 997TT AWD unit are great.
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Both are fantastic cars and both are a pleasure to drive. To each his own, drive what floats your boat. 
