K-mac and Whiteline have both selected their rear bar thickness on what they believe the majority of drivers/buyers will prefer in the back of their ST162. Thicker is not necessarily better or worse, especially on the rear, and it will depend upon your total suspension package and your driving style as to which bar works best in your circumstances.
If you are doing track work and don't mind adjusting your cars suspension (and by the sound of it, with coil overs and Koni adjustables, you don't) I would save up a few more sheckles and get the Whiteline adj. front sway bar and test and tune your ride on the fly.
I think Whiteline have some other trick suspension bits for the 162 as well, so I would be tempted to get it all from them in a package and ask for a lower price to boot.
Swaybars will always improve the handling of a car, up to a point where you are running complete track suspension with 500 lbs plus springs and Bilstein's, for example.