I own both a Model S and a Model 3. I've yet to find a single person (of maybe 50) who prefers the Model S interior. I've had gasps from how clean and futuristic the Model 3 is, but the Model S gets a "Meh" in comparison. The single, straight piece of wood along the dash with no vents is stunning. The center console is cleaner with better phone chargers and perfectly symmetrical. It has a better sound system, interior camera (currently unused, but still), and buttons to open the door (which feels more elegant than pulling a handle). Meanwhile, the Model S is playing catch-up with lighted vanity mirrors and MCU upgrades.
Of course, this is all subjective, but the Model 3 is simply more refined to me. This isn't to say the Model S isn't a beautiful car, but like
@henderrj suggested, it does make the Model S feel less like the Halo/Flagship vehicle. I don't say this to dismiss other opinions. Just sharing mine. I had originally planned to flip the Model 3 to a family member and replace my Model S with a new one. But now that I have the Model 3, I actually enjoy it more than the Model S. Even getting into a brand new P100D feels "dated" to me.