Without doubts GS is excellent and sportier, and seats are very good, while driving RX is like relaxing in a living room sofa, I already found easier to enter and exit the car.
The risk from who was a GS owner is to not like the SUV mission of RX. While GS 450h is really a fast car, RX is more a family car, much more space and relaxed driving its mission (not a problem to me, as it's faster than my GS 300h). Mpg are less than GS300h , but not dramatically different for what I see, and probably the same of GS 450h respecting speed limits ;-).