While my XC90 Momentum was often in for repair, I did have regular use of the managers R-Design. My recollection is I preferred the ride of the XC90 Momentum. Personally I would avoid low profile tyres if you want comfort and quietness. For me the R-Design distracted from the XC90's qualities as a comfortable car. Air suspenion with a low profile, over a steel sprung with a high profile, if you want comfort and quietness I would go for the latter.
As for quietness and comfort on the motorway, it would be close, but the RX would be better. The RX is much more refined in town and traffic due to the qualities of the hybrid drive.
I've owned previous Volvo's, and S60, and a XC70 from new which I was happy with. However since the takeover of Volvo by Geely if relaibility is important to you, I would avoid Volvo. That's backed by the customer satisfaction surveys and my experience.