This is quite honestly a very difficult question to answer, because ride comfort is somewhat subjective and it also depends on your typical roads that you drive on. It always has to be looked at relative to something you are used to driving where you drive. I've been daily driving a '13 RS5 for the past six years. I've done very long road trips, the longest one being 12 hours of driving in one day. From an objective standpoint, I wouldn't describe the B8.5 RS5 as a comfortable car, just based on what it is and that the suspension is a sporty firm fixed steel suspension. But subjectively it is overall more comfortable than the B7 S4 I had before and I have largely no complaints about it. I personally prefer a suspension that is on the firm side of comfortable.
I have extensively driven the B9 RS5 on some of my typical roads, including busting through some of my favorite canyon roads. It is more comfortable in Comfort mode than my '13 RS5 and harsher in Dynamic mode. I found the suspension spread to be excellent for a car in this segment. Compared to other models you also have to be careful, because AMG for example just reworked the suspension in the '19 C63S. I've only driven one on a relatively short test drive, but based on that, Comfort mode is very similar to the RS5's Comfort mode. I'll be picking up my brand new '19 C63S coupe tomorrow in 2 weeks. I'm doing European Delivery and will be driving it in Europe for two months. That will be the first real comparison to my RS5 on European roads. I've done European Delivery with my RS5 as well. The second test will be once the car gets here and I get to drive it extensively on my typical roads, where I've also driven the B9 RS5 and other current RS models. I'm expecting the ride to be better than my RS5 and comparable to the B9 RS5.
You currently have two cars at very different ends of the spectrum. The A6 is a plush ride in my opinion even with the sport suspension and the RS7 depends. I'm assuming you have the air suspension, so that's a different game. In terms of comfort, you can't beat an air suspension, because it can adjust spring and damping rate from very comfortable to technically very firm. Optionally, the RS7 is available with the DRC suspension, which is the same suspension as the B9 RS5 optional adaptive suspension, just tuned differently. The RS7 with the DRC rides noticeably firmer, but also flatter and better handling. The DRC's objective is not comfort. That's more of a by-product. Its main objective is chassis control. It diagonally links the front and rear dampers to minimize roll, squat and dive. It acts more like an adjustable sway bar and comfort is essentially dialed in by allowing more roll, squat and dive. However, as far as current RS models go, it's probably the most comfortable you can get. Perhaps not as comfortable as the RS7 with air suspension, though.
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