Well the purpose of a sway bar is to tune out oversteer / understeer on a track ... if you want one just because you want one, its hard for us to really make a recommendation.
Essentially though, a sway bar just increases the spring rate when turning, so anything better than stock will "help." Understand though, if your shocks / struts / sway bars aren't set up properly you could introduce a fair bit of over or understeer depending on how you do it.
I typically find when running a square set up that RWD cars prefer a little more front bar when cornering (or at least that I prefer a little more front bar with how i set my suspension up). So, im not going to tell you what to get, i'm going to make you do some homework based on my suggestions here, but i would either just get a front bar and set it on a mild or low setting (probably the easiest to adjust and live with) or if you want front and rear bars, just make sure the front is set stiffer than the rear (more involving, less potential for improvement if you're not going to actually track and mess with it).