Roll / sway bars have nothing whatsoever to do with traction.
Roll bars stop body roll through corners. As a rule-of-thumb, stiffer front roll bars increase UNDERSTEER, stiffer rear roll bars increase OVERSTEER.
The stiffness of a roll bar is also proportional to the diameter of metal in the bar. So a 27 mm HOLLOW bar may be lighter (less stiff) than a 24 mm solid bar. I found this when I replaced a 25mm (hollow) rear bar on my GTS4 with a 24mm SOLID bar.
You need to check all the properties of the bars in question, and decide what you want the bar to do, before it can be determined which bar is best. That said, Whiteline has done a heap of work working out the best combo of bars (and other suspension components) for these cars under Australian conditions.