Keep in mind that - For most Off Road racing events that are not "Ice Races", screws are generally illegal (mostly because they will pull out, and be left on the trail). Screws are for pond/lake racing only.

Screws = Pond/Lake riding
Rally Studs (long, short, auto) = Trail riding

There is a huge difference in cost and application. For Rally Studs on sale, see: Kevins Discount Motorcycle Parts