In sway bars and spring replacement it is easy to do as much harm as good. You might like one brand in front, a different in the rear, and totally different choices for 16", 17" and 18" wheels.
There is no one perfect setup for all uses. Watkins Glen, Oval track, last weeks autocross and your daily commute would all have different 'perfect setups'. I recommend you drive a car of an owner who has a 'perfect setup'. If it turns out to be perfect for you, then buy the EXACT same thing. If not, keep looking.
Unless you want to proudly display a sticker, Brand name, used vs New is of relatively low importance for sway bars. If you can define your parameters, the Boxster Racing Board
http://www.iq.dynip.com/~racing/index.cgi/index.cgi? may have suggestions. Nothing beats driving the setup you are considering installed on the same model and trim level as you drive.