My model S is a 2013 and is a fairly base model. Personally, I really love this car, but for my wife it's just too big and she'd prefer a smaller version. When the 3 is available, she's definitely getting one (unless they're god-awful ugly like some of the renderings). The new 3 will have so many bells and whistles standard that my S doesn't, and possibly (probably?) features that even the new S's don't have yet. Technology is always improving and typically getting cheaper, so the 3 could be a technological marvel when it's released.
But unless the jealousy of my wife's high-tech Model 3 gets too much, I'll keep my S just for the sheer comfort, and because it is a larger car. Besides, it already puts my Camry's to shame. I'm happy!