Mine is the 2004-2006 S1 model.
I've not ridden the S2, but, from what I've read, it was an improvement with better brakes (4 pot, rather than 2 pot sliding calipers) slightly better engine performance and a few tweaks to the instruments (analogue rev counter, rather than LCD) and rear foot peg hangers (bolted on, rather than welded on).
Probably, if I was getting another one now, I'd go for the S2, but if an S1 came up at a good price, I wouldn't turn it down.
I've not ridden the S2, but, from what I've read, it was an improvement with better brakes (4 pot, rather than 2 pot sliding calipers) slightly better engine performance and a few tweaks to the instruments (analogue rev counter, rather than LCD) and rear foot peg hangers (bolted on, rather than welded on).
Probably, if I was getting another one now, I'd go for the S2, but if an S1 came up at a good price, I wouldn't turn it down.