I have an 18mm sway bar from SPD - the 18mm bar works great - and is recommended as the bar to "begin" with.
18mm bar = understeer (front tires lose traction a bit and car tends to "plow"
20mm bar = oversteer (rears lose traction and rear end starts to come around on ya)
Basically it depends on what type of driver you are. If you are a spirited driver and are used to the car getting loose on you, maybe the 20mm is best?
But, the 18mm does indeed stabilize and make the car very easy to predict.