as said the softer compound one can take more abuse, but it is the one that can graze if hit against a kerb, what you have to remember is you actually have one on the car.... so make sure you dont hit a kerb, dont drive straight at one! if you need to get up a kerb go at an angle.
Friend has put a seat Leon Cupra R on his 206Gti and that doesnt go near kerbs, think of the front splitter as a GRP front bumper that will smash to pieces if it touches a kerb (that should stop you from wanting to get too close lol)