I have noticed heavy differences regarding not only his features (-> I mean the colour of his eyes, the shape of his lips etc.) but also his expression - that is even more important, I believe.
A portrait like Hamel's (http://saint-just.net/art/hamel4.jpg) shows a young man who looks severe and austere at the utmost (it reminds me of some kind of German priests !), while the well-known David's features a man who has a vein of melancholy and sadness in his eyes, an air of gloominess which surrounds him someway and suggesting a kind of introverse, withdrawn personality. If we take Greuze's portraits we find a rather cynical dandy and Prudhon's is something further more different... All this things despite the portrayal of his strict physical appearence !