The USR rotors are hard to come across. I got my USR923 pair of rotors more than a year ago, got an incorrect shipment (GD923), then a month or so and some phone time later got the USR disks.
The major advantage given by EBC for the USR series versus stock is even pad wear.
The GD series may have more noise compared to the USRs. Because the slots extend to the edge on the GDs, there's some cooling afforded by the limited airflow under the pads. The GD series probably has more grip due to the added edge area from the dimples.
Since I was more interested in "quiet" and something to avoid problems after going through a puddle (wet pads/rotors can reduce braking horrifically) the slot-only design was my preference. When I originally saw them on a canadian site, the USR description incorrectly suggested they were black "all the way through" as in "carbon loaded ceramic" or some such but further research proved that wrong. They have a black coat but standard brake metal underneath much like the GD series is gold from the coating, not all the way through.
Gold or black will still show at the edges and the hub though I haven't seen mine for a while thanks to the kleenwheels dust shields.
I'd suggest you get the GDs (GD3, whatever) because of the availability. I don't think "noise" is a real issue for anyone with these rotors in a Smart but I'd happily defer to experience.
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