The skis companies call race stock are usually stiffer, more demanding and more hard-snow specific than the regular race skis. They are the same skis used by (entry level) FIS athletes and are designed to meet FIS regulations, while the regular race skis do not keep them in mind, although most of the regular slaloms would be fine for FIS. The race stock GS will be much straighter than the non-stock versions.
How much do you weigh? How many years have you been racing? What are you skiing on right now?