well, for one, WRC uses the real courses you'll see on TV, during the WRC broadcasts, even if they're a bit shorter in the game. then there are the cars, which aren't all in GT3. and there's damage, varying climatic conditions, night and day racing, etc... it's just a better rally simulation than Gran Turismo.
but WRC's problem is that it's too short. i'd suggest you wait for WRC2 or Colin McRae 3, both of which look promising and better than GT3's rally mode, which, while better than GT2's, is still lacking.