I've got the ProMINI strut brace, which has an off-centric mounting hole at one end to pre-stress the bar. I found once, after having some work done, that it wasn't correctly tightened. After tensioning it up I noticed a big improvement to the mid-corner precision and the solid feel of the front end (note that I've also got the early JCW coilovers and do motorkhana/gymkhana/autocross mine).
So my theory is that the diff. between the bars is more to do with "is there any slack after installation?" rather than "is one bar stiffer than the other?".