My experience is that the Giottos are far more delicate than the GGS. Every Giottos screen that I have had (3 or4) have cracked, chipped or scratched. I've had a similar number of GGS screens and none has had any damage. Perhaps it is just bad luck, but I see a trend.
In addition, the SP8304L, which I bought with my D800, covers part of the screen with the wide border (as noted by Dave) and is not as good of a fit. It is really a Canon sized cover.
I will say that the Giottos screens are slightly for viewing than the GGS.