S1 rotors are smaller in diameter, but vented - thus they can dissipate the heat build up much more efficiently than the standard solid rotors (there's only 4 mm difference). the S1 calipers are 18 mm thick too, compared to (off memory) 13mm. The S2 rotors are 258x22mm, so 20mm larger in diameter, and 4mm thicker. Grip is determined by your tyres, but yes, as the brakes suffer from heat effects less, you get a better stopping consistency over the standard solid rotors, and also lessen the chance of glazing the pads, as the heat build-up is much less... and to a lesser extent, because of the heat dissipation, you can modulate the brakes much easier - although doing up in diameter as well, makes for a more noticeable difference when it comes to modulation