The anx1 is an analog style physical modeling synth, it was patterned after the prophet 5. the CS2x is a combination of synth types(physical modeling, ROMpler). Yamaha has a plug in board that does essentially the same thing as the anx1 that can be installed into the CS2x. The plugin board has less polyphony(half that of anx1) but otherwise is the same as the anx1.
plug in board price =$240(us). Only problem is that the plugin board doesn't have knobs, you have to adjust everything through software. Knobs are really nice to have on an analog synth. The anx1 was/is a really nice piece of kit, I don't know why Yamaha discontinued it. I'd probably get the ANX1(if it were me) cause I already have a nice ROMpler style keyboard.

DudeManCentral