I much prefer the ride quality of the XTS over the DTS. The steering feels more controlled and the suspension more refined. As for the 20" wheels, I read the negative owner comments here as I did my pre-purchase research. Some of it was valid, some not, -for me.
The 20" wheels have an inherent manufacturing flaw. If the chrome spoke inserts contact the painted aluminum rims, a chemical reaction (galvanic corrosion) takes place between dissimilar metals. The nominal gap between them should be .050" when measured at the outermost edge. Otherwise, a spidery-looking stain will form as the metal etches. Two of my wheels exhibit this problem, and two are fine. My dealer has offered to replace both under warranty.
The ride quality is excellent and road noise is minimal --on smooth highways. This is true for most tires. Due to the low profile OEM tires, a pothole strike can be catastrophic at speed. The wheels are prone to yielding depending on the severity of the impact. If you drive on roads where potholes are common, this is an issue to consider. In this case, the smaller diameter wheels with higher profile tires are a safer bet.
One of the most highly recommended tires here are the P7+ and I'll migrate to those when my current tires (OEM) are replaced unless something better comes along. Positive reviews by satisfied customers are important to my decision making process when buying products that I can't always test before buying.
Enjoy your new car!
CC