The difference between MOOG CK and RK is that MOOG CK is the premium line of MOOG products.
And RK is Moog's value product, but often with OEM quality.
MOOG CK parts are engineered to address any potential OEM design flaws and offer ball joints that are also serviceable, and RK parts feature ball joints that are non serviceable.
MOOG CK parts are designed to correct any potential flaws in OEM designs and the ball joints with MOOG CK are serviceable.
MOOG CK may even be engineered to be better than OEM specifications and often have a limited lifetime warranty.
MOOG RK are the value product line but often with OEM quality and the ball joints are not serviceable, and MOOG RK parts are designed to be equivalent to the factory specifications and often have a shorter warranty than MOOG CK.
Both CK and RK MOOG products and parts are high quality although CK MOOG parts are often more robust and better especially when addressing any potential OEM design issues.
RK MOOG parts are a more cost effective option and are suitable for people wanting OEM quality.