I have both an Nx300h (my wife drives it now predominantly) and Gs450h. I can tell you I regret getting the Nx300h.i was in a similar situation to you last year, with a budget just under 30k debating between a used rx and nx. If i have the chance to do it again, I would have gotten an older generation rx450 with air suspension. Much better ride comfort/quieter cabin, during the test drive. I can’t comment on the fuel economy difference between the rx and the nx. However, between the Gs450h and the nx300h, the nx was better averaging about 38-40mpg compared to early thirties in the Gs.
The reason I didn’t get the rx450 was only simply down to the fact that my wife couldn’t live with the looks (i am referring to the pre facelift models (<2015), which I admit, the nx has a much more palatable styling and interior.
hope it helps.