I've thought a lot about this too (June 2016 purchase).
When you add AWD, Autopilot, bigger batteries, non-black paint, non-aero wheels, Supercharging, etc etc, the price starts to look a lot more S-like. Even with the premium enhancements, the 3 is 20% less car (and not aluminum either) than the S. If the price is about 35% less money, this is still phenomenal progress by Tesla to achieve this -- well, we won't truly know what they've accomplished until their 2018 Q1 earnings report -- but you are getting less car.
If I was buying new today and the 3 was available to me right now, it would make for a very painful decision. But since it isn't, and I am not, and given the points above, I don't have any regrets.