i'm not claiming to be an expert but i think the wrx is decently balanced stock. i personally went with a rear sway and kept the front stocker. my reasoning being that the car, however balanced it might be, tends to understeer. stiffening up the rear end helps to take some of that understeer away. lots of people go for both front and rear sways though, i'm assuming they are trying to keep the car more balanced.
the front endlinks are metal whereas the backs are plastic. you definitely need rear endlinks with an aftermarket sway, but not necessarily front ones....not that it would be a bad idea.
like i said, i'm no expert and i'd get more opinions from some of the more senior members here.