Tough choice. I’d personally pick the 2020 model 3. It should have the second generation Intel CPU for the infotainment system, plus it should have HW3 for better autopilot behavior.
Plus, the 2018 Model S is now two generations old on the powertrain, so it’s likely that the mechanics only get minimal training on those older systems. And the battery uses older chemistry compared to the model 3.
That being said, you’d get a ton more space with the model s, and it’s likely going to be a good car.