The Saiga and the PSL are designed for 2 different roles. The PSL is designed to be an actual sniper / designated marksman's rifle. As such, it's got various reinforcements designed to accurize the weapon. They can typically shoot around 1 MOA. The Saiga .308 is designed to hunting accuracy. It was reinforced only as far as necessary to effectively handle the .308 round, and typically gives a 1-2" pattern, depending on gun. 1 MOA Saiga .308's do exist, but they're rare. Pricewise, they're about the same for a basic Saiga .308 (not the deluxe model with the thumbhole stock and proper trigger group). The PSL is $750 from AIM Surplus. Basic Saiga .308's WERE $550, but they've gone up quite a bit. You have to convert a Saiga .308 to get it to look and feel like a PSL, though, or get one that already has a forward trigger group (the deluxe thumbhole stock version) and swap the stocks.
Pro's / Con's:
PSL: It's a slightly better rifle. More accurate. Significantly lower recoil due to muzzle brake. Lighter weight, as well. Takes SVD-bracketed scopes. No 922(r) compliance problems as it's exempt as originally imported, and you have no reason to change it.
Saiga .308: Widely available non-corrosive American ammo. Takes AK-bracketed scopes. You have to convert it and fit a brake to make it comfortable to shoot. Polymer mags. Have to deal with 922(r) compliance. More expensive when all is said and done.