I'm runnin whitelines on my s13 too. I have three way adjustables up front and two way in the rear. Because of the stock suspension set up, anything stiffer than stock in the back will make you oversteer. To pull a g on the skid pad (meaning trying to find grip) SCC pulled the rear sway bar off and adjusted their whitelines adjustables to full stiff up front. They hit their goal of a g, but keep in mind that it was achieved power on.
So, to my point, you run anything stiffer than stock on the rear, you're gonna set the back end free with oversteer. But the question is, how much oversteer is too much for what you're doing...
The rest is up to you.
... but to help out just go here
http://www.pdm-racing.com/products/suspensionbars.html
Some people will steer you away from Whiteline's because they don't like the linkage on the ends. But it all depends on how much you care about stuff like that. I love mine, and they come with polyurethane bushings designed specifically for Whiteline sways. They aren't the thickets out there, but I don't track my car, and even if I did, the Whitelines at 27mm are just fine.