It's all in the details. The GT S is a bit lower and has a few more blingy bits on the body like fog lamps, a mesh grille and multi-spoke 20" wheels.
Here's a normal Quattroporte:
http://en.wikipedia.org/wiki/File:Maserati_Quattroporte_-_2.jpg
And this is a GT S:
http://en.wikipedia.org/wiki/File:2009_Maserati_Quattroporte_Sport_GT_S.jpg
There's also a couple of interior revisions on the GT S which include Alcantara and carbon fibre accents. It also has upgraded iron/aluminum brake rotors and the engine has been fettled a bit to produce about 433hp.