Big differences. The PSL is a modified AK, the Dragunov is a different rifle, kind of an evolution hybrid of an AK and an SVT. It has a short-piston gas system on a milled receiver using an evolved AK trigger/safety setup.
They serve an identical role and are similar in accuracy, though Drag owners will say the SVD is more accurate. I'll grant that, but the PSL serves the same purpose in combat. I would wager the PSL is more rugged. Yeah, the SVD has a milled receiver, but the gas system is more fragile (but not fragile). That said, the PSL does not need heavy bullets fired from it regularly.
In any case, in a practical world, they are the same. Nobody should expect bolt-action accuracy out of either one (no, you shouldn't). At this point in time, the PSL is a better bang for your buck. SVD's are very expensive, even the denuded sporter versions.
Ash