When the front slides (pushes), that's understeer. When the rear slides out, that's oversteer. A thick front bar will result in understeer and a thicker rear bar is required to correct that, and vice-versa. Most cars have understeer "programmed in" from the factory, as less experienced drivers can usually correct a front end slide better than a rear slide (spin).
BTW, I updated my sway bar spreadsheet and PDF file today. The bushings for the rear bar have been superceded to a MUCH cheaper W210 part number. I also have a new part number for the E60 bar, but don't know if that can be ordered in the USA (yet) :
http://www.meimann.com/images/mercedes/W124_sway_bars/