The Model 3 is still 2 years out! He said end of '17, but that's probably Tesla time. And even if it does happen, it's likely to be very limited deliveries to meet the promised timeframe. I think there's a reason hardly anything was announced - because in 2 years, it's all going to change! The only reason to wait on purchasing a model S is if you can't afford it. For 35k, it is not going to have the features of a Model S. Once you start adding options, the price is probably going to raise quickly. As shown yesterday, the 3 is smaller, slower, only has one screen (no instrument cluster display), and the interior is nowhere near as nice. There is also 2 years for the Model S to continue to improve. Who knows what features could be added to the S in that time. A body refresh? Auto open and close doors? Cooled seats or overall improved seats? New displays or processors to power the displays? Improved batteries and range? Stuff no one has thought of? Any improved battery tech powering the 3 will likely make its way to the S, possibly before the 3 is released. There are so many unknowns right now and I see no reason to delay a Model S or X purchase (unless you think big changes are coming soon to the S, which is an entirely different discussion). They're just not comparable. You wouldn't debate holding off a BMW 7 series purchase to see what changes are coming to the 3 series!
If you want to wait 2 years (or probably a lot more than that by the time you get the car), then go for it. But it shouldn't be because you want to decide between a premium vehicle and a mass market one.