Panhard bar, or rear sway bar. I would think the sway bar, but I'm not sure.

 

you're comparing apples and oranges, they each have drastically different functions. Panhard rod keeps the rear axle from moving laterally (side to side) during cornering. The stock one is a piece of stamped steel with rubber bushings, and can (and does) deflect (bend slightly) under hard cornering loads. Aftermarket panhard rods are made of much sturdier tubing, and have vastly superior bushings, so less deflection and more stable cornering. You can choose between adjustable and non-adjustable, solid bushing, poly bushing, and just plain rubber (but firmer than stock) bushings. If you've lowered the car in the rear your axle has shifted slightly off center due to the lever effect the panhard rod has when it moves up and down in its travel. an adjustable one will allow you to re-center the rear end, but mind you the offset if very slight, hardly noticable even with a non-adjustable one. Solid bushings are loud, require regular lubrication, and wear out quickly if you drive on the street a lot, but offer absoultely no deflection under hard loads, thus giving you the best lateral stability. I would suggest unless its a seriously prepared road race or modified autocross class car going with the poly bushings, but they also need occasional lubrication (you'll know when that is, they will squeak)
Stabilizer bars increase roll stiffness of the suspension. A bigger bar will let the body lean less in a corner, but in the rear of an F-car a big bar is a liability. Our cars basically oversteer at the least touch of the throttle anyway, and a stiffer rear suspension will make that worse. Larger front and rear bars, in a balanced combo, will make the car handle MUCH better, but if its a choice between the two, a bigger front bar will yield much better results. I run a 35mm front bar and finally am satisfied that the car has sufficient low speed roll stiffness.
sway bars, by the way, will not make the car ride harder on the street, in a straight line they add nothing to the spring rates. The worst you might experience is a bit more road "feel" if you put harder bushings and end links on with a bigger bar (again, poly is recommended)
Strano Performance has a great deal on a front/rear bar combo