Plastics are subtly different. The cuff on both is PE IRRC.
The RS engages ever so slightly quicker than the RX. RX has a longer engagement range and is designed more around off piste skiing. This will lend many people to perceive the RS as a stiffer boot.
Liner on the RS has more volume and denser padding. Easily one of the best stock liners I have ever skied. I was impressed with the RX liner as welll, has volume where you want it, super thin through the sides of the forefoot for great shell contact. RS liner does have a single laminated cover which will increase durability compared to the RX. This would only be an issue for the 60-100 day a year skier.
Both boots use the dual core mono injection construction which does deliver an increadably smooth and progressive flex. Overall the RS 130 is the best ski boot I have ever skied when combining comfort and performance. For a mortal like myself the performance between it and a Head RD raptor (plug boot) is negligible.