FYI...for what it is worth...my daughter-in-law has a 2007 RX350, and she thinks the ride is very good. She has her car serviced at Lexus, and they put her in latest model RX350s. Her comment is "I wouldn't buy the new one, too stiff/rough compared to my 2007." I drove one of her loaners and agree it is "rougher." Noise a little noisier as well. She has ridden in my MKC and agrees it is a better/quieter ride, compared to both RS350 models. Mine is usually set to "normal." The "comfort" setting is very soft/cushiony. Over the years it seems Lexus is changing their ride to match their look...more sporty/edgy.
I cross shopped and test drove an RX350 F-Sport (the NX wasn't out yet). I have to say I wasn't a fan of the handling. The F-Sport is supposed to be more taught than the base, so I'm not sure how that compares.
I like a sporty firm suspension and a planted feel. My wife likes a more pliant ride. The MKC was able to accommodate us both with it's sport and comfort modes (we have the AWD with CCD, I didn't test the FWD MKC). The RX felt somewhere between Comfort and Normal to me (from the MKC). A fair bit of body roll and dive under braking, but it is a bigger vehicle than the MKC. I was pretty surprised by how numb the steering feels (and light). I didn't notice much of a difference in sport mode in the RX. It definitely doesn't change as much as the MKC does between comfort and sport.
So to conclude, if you like a very smooth ride I would say the MKC on 'Comfort' with the AWD CCD is a more compliant and smoother ride that still provides more road feel through the steering. I also think it is probably a bit more quiet inside, but that might be close. I do think the MKC engine can be heard more than the RX. The active noise cancellation in the MKC really works well to filter out those lower tones, but there is a degree of higher frequencies that can be heard. I wonder if that tuning is intentional to allow drivers to hear sirens?
That being said, I'm rarely in comfort mode unless my wife is driving so I guess take whatever I said with a grain of salt and the obvious bias considering which one I purchased.