I'm sure the CSR is newer, they weren't available when I had my R series polks, as far as the difference, I'm not sure, probably just made to match the newer R series speakers. I'm sure the guys at the polk forums would be better suited to answer those questions.
Just save up and get the best you can get, cause once the upgradeitis sets in, it's hard not to keep buying stuff... I started out with a low end Polk R series setup, and now I have top of the line def tech and SVS stuff, and it wasn't cheap, but what can I say, I'm an audioholic