Idunno, Metro 2033 doesn't seem that vastly similar, I think it's supposed to be a lot less open-ended than STALKER. I still think you should get CoP, forget about CS, and just rush straight through the plot or something, or just read a spoiler, the ending isn't that impressive, but it's kind of handy to know about while you're playing CoP since Clear Sky gets brought up a couple times.