I have nothing to back this up, other than the “seat of my pants,” but I have always felt that ride quality, especially in a luxury sedan, was better in a RWD car, compared to an AWD version of the same car. With all the additional weight of the components needed to drive the front wheels, it just made sense to me. In the case of an all electric vehicle, such as the BMW I7, where the RWD has one motor and the AWD version has two motors, it would seem to me that the RWD car should have at least a different feel to it and possibly a much better one. I know that AWD is better in the snow and other inclement weather conditions, but other than those instances, doesn’t RWD offer better ride quality? I know that I much prefer the ride quality & driving dynamics of my RWD S560 vs my departed S580 4-Matic, but I’d like to hear the thoughts of others here.

AWD vs RWD does not affect ride quality. Driving dynamics and steering response yes, but not ride quality. The latter is all in the suspension. Two cars, everything the same just AWD vs RWD would have the same ride quality. However, manufacturers don't necessarily tune the suspension the same for the AWD vs RWD models and in your case you are comparing two entirely different generations. Since there's no S580 RWD you can't do a like for like comparison.

I’ll take your word for it, but I still don’t understand. With all that mass of additional weight over the front axle, how can it NOT affect ride quality? I bought my 2019 SL as a 450 with the V6, not because I didn’t want a V8, but because with the lighter V6, the car rides and handles SO much nicer. I test drove the V6 & V8 models back to back multiple times and could not believe how much better the 450 drove. Virtually every magazine review came to the same conclusion: the lighter V6 model was the way to go. When comes to ride quality, why isn’t this the case with AWD vs RWD?

Lots of factors to cover here, but not in the mood the write an essay, so let me try to summarize a few things. AWD is essentially a front diff + shaft to the transmission. The additional weight on an already heavy car is marginal. A V6 is not only lighter, but also shorter. 3 cylinders per bank instead of 4, so the engine can be placed further back giving a better front to rear weight distribution. You are ultimately right that when you sum up all at the individual parts then it may make a difference to the ride quality, because all these changes often lead to a different tuning of the suspension depending on their objectives. For example as AWD vs RWD is concerned. With AWD the front wheels have to do two things. They have to steer the car as well as propel it forward. That means the available grip of the front tires (finite) has to be shared between steering and propulsion. The consequence of that is that AWD cars tend to understeer, because the forward propulsion takes away lateral grip. Same for FWD. With RWD all the grip of the front tires is available to steer the car, so the front end sticks much more. Car manufacturers can play with the stiffness of the sway bars for example to compensate for the additional understeer tendency in the AWD cars, but stiffer sway bars means less ride comfort.

So, in summary AWD vs RWD by itself does not affect ride quality or at most marginally. What affects it are the suspension changes the manufacturers may decide to do to deal with the disadvantages of AWD. However, it's not just AWD vs RWD that can affect suspension changes. Same happens depending on how many options you add to the car, because everything ultimately adds weight. The curb weight of a specific car is just the base configuration. Manufactures typically use a few different spring rates etc. for various configurations of the same car depending on how it's optioned. So in principal you are definitely on to something, but it's the sum of differences.

Yes, the physics are as you describe. Higher unsprung mass with AWD from the presence of front halfshafts means less ride comfort than RWD. The effect is small and arguably not noticeable.

Car companies can tune most of this difference out by choice of wheel, tire, shock and spring specifications, or programming in the case of adjustable suspension.

I also liked my RWD S550 ride much more than my S560 4-Matic. I already knew that in advance, but where I live, I really need the 4-Matic. My first W222 was a RWD and it rode better in my view despite the engine being less smooth than the facelift which was the 4-matic one.

The front Control Arms / Bushings are going to be different for AWD than RWD because they have to react the driving torque. This can impact road vibration transmission.

I had a AWD and then a RWD LS460, the RWD one was an LWB and had air suspension while the AWD was on coils and was SWB, so its hard to say if the ride was any different.

I did test drive both a RWD and AWD S560 before I bought mine and didn't note any difference in ride. I do prefer how RWD cars feel, but thats not really a ride thing.