So should I only get that one and not the other one? Thanks for the helps!
Call of Pripyat is the last game released, by far the most polish and accessible. Also, the best overall.
Shadow of Tchernobyl (strangely, the most expensive) is the first one. One could argue it's still very good, and I would agree, but it's not as good as CoP. If you like CoP, try to buy Shadow of Tchernobyl one day, because the last two hours of the game, especially if you get the " good " ending, is one of the most awesome fps experience in my gamer's life. Also, for your culture, Shadow of Tchernobyl is now a cult classic, a breed of FPS that has disappeared.
Clear Sky is a prologue, and even if it introduce the base of everything that made CoP good, it's still a buggy, unbalanced mess with great ideas but a terrible execution and a very disappointing end game.