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.