The S&S has less friction (Skar mentioned it was more "noisy", this is probably why) and more "feedback" when you're moving your mouse across it. The 9HD has a little more friction to it but both are decent hard pads. I haven't experienced any tracking issues on either but I really only used the S&S with one mouse.
Just be aware that SteelSeries decided to use the worst kind of rubber backing on the back of the S&S (those rubber paint splats that you are referring to). If you play on a wooden desk, the mouse pad will not stick at all and will constantly move around. If the surface of the pad doesn't really matter to you I would just pick the 9HD.