[Flieger]

The thing that changes the handling balance is not the absolute stiffness (assuming a reasonably smooth road) but rather the distribution of roll stiffness between the front and rear. More precisely, it is the lateral weight transfer that causes the track to loose cornering power. More lateral weight transfer at the front means understeer. More at the back means oversteer. Your total weight transfer will be essentially the same no matter if your car is stiff or if it is Cup Car stiff (body roll will cause a shift in cg to the outside of the turn that will increase the weight transfer if you make the car really soft). So it is all about how you proportion that between front and rear.

In a pure turning situation (no pitch, no bumps) sway bars essentially do the same thing as the torsion bars in that they both add elastic roll stiffness. Therefore, you don't need either or both on all cars, as the other thing to consider is the roll center height at each end, as this adds geometric roll resistance and reduces the roll couple that the elastic elements need to react. (In the more extreme cases, such as a swing axle on a formula vee, you might see no traditional rear springs at all; only a "Z bar" or "heave spring" that is the opposite of a sway bar and works only when both wheels move the same direction. The roll center height at the rear is plenty high enough for a car with such a low cg that there is no need for any elastic roll resistance.) There would then only be one anti-roll bar at most: an adjustable one at the front.

For a 911 you get the front to rear ride frequency ratio right and your pitch and heave stiffnesses where you want it, then you get the roll stiffness you want with anti roll bars (preferrably adjustable). I don't encounter too many bumps that hit both wheels at once, so to me there isn't any real reason that "big sways, soft torsion bars" should have a better ride than the opposite if you are assuming the same roll stiffnesses. On the contrary, the big sways will transfer weight faster at the instant of a bump on one wheel, meaning that handling, if not ride, will be worse with the big sway bars. I also don't like the pitch to be more pronounced than roll. As a result, I would ideally spec the rear springs a bit stiff and use only a front anti roll bar to tune handling. This gives more ground clearance out back and reduces weight a bit. It also helps get the rear ride frequency up (since lightened 911s typically end up even more rear heavy than stock) which provides a more level ride. With torsion bars, however, you can't always get as stiff as you'd like. Hence coil-overs.

I can't give you good rules of thumb for an amateur. For that I would tell you to talk to racers in your area or else suspension shops like Elephant or Rebel. I can tell you that stiff torsion bars means less sway bar, if you want to hold roll angle constant.

Pros calculate what the elastic weight transfer and geometric weight transfer is for each wheel based on the roll center heights, spring and sway bar stiffnesses, cg height, track widths, etc. And this ignores dampers which are responsible for a lot of the transient behavior on turn entry. The pros will then look at their tire data and determine what the optimal distribution of that weight transfer is to give the best balance. Most of this stuff is not what an amateur would have.

For 911 club racers I would say go as stiff as possible on the rear torsion bars, get something that matches in the front, and use an adjustable anti-sway bar at the front to tune the handling by making small changes throughout a session until you find what you like.

Remember, too that stiffer rear means less weight on the inside tire, which means more inside wheel spin and more need for a limited slip differential.