I've tried both of these balls, and to be honest, 9 out of 10 golfers won't notice the difference between them in terms of feel & distance, but it's the spin around the greens where the rxs is better (i think it spins too much for me!). They are very similar in their performance, and they are both aimed at golfers with a swing speed of <105mph. It boils down to whether you want more spin on approach shots or not i suppose?!?
I also play the RX (and some left over e6 for those whatever days) and I love the RX. It's one of my favorite pieces of golf equipment really. I would like to try the RXS and I'm hoping I'll get a box for Christmas - I don't want to cheat on the RX hahahaha. Just kidding. I'll probably wait until next spring to try the RXS so I have good weather to play in.
I've yet to try the RXS, maybe I'll try some today. But I think it just boils down to what Doolal stated. More spin, less spin. I think the RX is a great ball! If I weren't on staff with Callaway I'd definitely be playing the RX.