Fundamentally they are all the same. The PSL-54C is the newest incarnation. Have a care that you get a newer one or that you are looking at lacks the "third hole weld" that means it is contraband. Century is supposed to have gotten them all back, but you never know. Any PSL-54C with a 2007 date on it or later is fine. The advantage of these rifles is that the PSL-54C is made up from original military PSL's. Their bayo lug has been neutered, but it is still there. These are neat because they have the IOR-Valdada scopes which were original to these rifles. Their reticules were tritium-based and almost certainly don't glow anymore, but they are the equivalent of a military scope made by Leupold.
Any marked Romak-3 will be good, too, but likely will have Russian optics which have a battery-operated reticule. Some of these will require a pretty obscure battery that is available but by no means at your local Walmart. Any SSG-97 from InterOrdnance will be good, too. This group is largely commercially-made and as such may be in the best condition. The SSG's would need a 1990's date to fit in this slot. Later SSG's may be parts rifles like the PSL-54C's. Most of these will lack the bayo lug.
The third and least desirable group are the FPK's assembled by Tennessee Guns. These are parts-rifles like the 54C's above, but instead of being assembled at the original factory they are assembled in the US. These are generally fine rifles and there are many happy owners of them. However, there have been reports of less-than-good assembly and some owners have not been happy. Chances are pretty good, though, that any FPK marked rifle will still be a fine one and I would personally not have a problem with owning one.
(And before some SVD enthusiasts pipe in, no, they are not REAL Dragunov's, but they are certainly Dragunov-style rifles that accomplish the same thing in the real world.)
Ash