Internally......different coils to a series one. turbo has a plastc compressor wheel (series one has a metal one), both have ceramic exhaust wheel. Engine is the same and so is the gearbox etc.
Or if you mean internally as in Dash etc, the S2 has a nicer looking airbag steering wheel, dash is the same but surround is slightly different (the platic is more textured and the coin holder on the right of the steering wheel is gone).
I am sure there is some more but cant think of it all right now. I think that the S2 is worth the extra $$ for the updated front end.