i never really liked SP cuz i was never that into the whole stealthy thing... i played SPPT because the multiplayer was a freakin blast (four player lan ftw).
chaos theory, however, is hot. they added so many sweet awesome moves to the single player that it makes you feel like a complete ninja :ph34r: . then, when you're done with single player, you can move on to the coop (which as we all know, ANY sort of gameplay is boosted 98787765 points when it's coop), which is really cool with all the coop moves you can do with eachother, including timing certain attacks or totally owning a group of 4 terrorists with ambushes. and after all that, you can move on to versus, which is even sweeter than the versus in SPPT because they added so much.
in short, you should get chaos theory when you're done with the first two. i really like it.
no, i LOVE IT <3