It is a myth that short barrels are less accurate. Like Dale said, short barrels have less velocity, more muzzle blast, and recoil more (less weight). These are dependent on the cartridge being shot, powder being used, bullet weight and how short you make the barrel. On the plus side - A short barrel is generally more accurate (less barrel whip) and lighter/handier. I would think that a 308 SPS would be plenty accurate and a 20" barrel is a good compromise between a really short barrel and a more standard 24" barrel.
BTW No matter what brand you are buying the police/tactical models are usually more accurate than standard stocked rifles, not always.