One of the reasons i bought an s2 over the s1 besides the fact that i think they look better in stock form are some of the extra features like dual airbags, abs, smaller nicer steering wheel, more robust seat trim etc.
I think the s2 generally looks a lot more refined and not as dated with its shaper headlights and bonnet ridge, the stock fog lights in the front bar and the rear wing looks better than the s1. The s1.5 is not a bad option, bigger front bar and rear wing looks good with a nice grille.