Hello1234: I do wonder though, what's really the difference between the games, they all kinda look the same honestly.
Basically:
SoC is the original, containing the perfect balance between freedom and linearity, horror and manshooting, emergent content a pre-scripted events. It's basically a mix of its own sequels (and one of them is actually a prequel. Ugh.).
Clear Sky is the most sandboxy of them all, relying on unscripted AI to provide as many diverse situations as possible. It's big, it's open and it does sandbox gameplay reasonably well. Probably the biggest disadvantage is recycling of areas from the original.
CoP has the most videogamey structure out of these games and game's progression is the most traditional. It's a bit more linear than SoC was, but the pre-built content is also a lot more interesting and polished. While I believe CoP to be the least replayable out of the three, I think it's the best one for a first-time player.
As for features, they are mostly the same, but further installments do generally offer more stuff - most notably better weapon and equipment upgrades, weapon repairing and so-called blowouts, which ... Well, you'll see.