I find CS to be a lot more linear (or at least, not so dynamic) than SoC. Once you clear out a region, nothing happens there. :/

It's been awhile since I played it, but I seem to remember impromptu firefights and mutant attacks breaking out all the time, and areas constantly switching hands.

I haven't seen a single region takeover so far. Mutant attacks and fights between factions happen, but always at the same places, it gets kind of ignorable after a while.

Here's to hope that the third game will go on sale at GG soon.

That level of randomness is one of the reasons why I considered Stalker similar to Vietcong - and love both of them. Vietcong may not be as random as Stalker as it's a linear shooter but the AI is often just as unpredictable, disappearing from the last location where you've seen them and reappearing somewhere completely else. It's funny that there's many gamers and even professional reviewers who consider this kind of behaviour "bad AI". I call it "disturbingly authentic".