"Is the new Jetta on the same platform as the old one?"
The Jetta & Golf got a new chassis this year, as well as a couple of new engines. Measurements of the new Jetta just about match my '93 Passat. Gone is the old twist beam rear axle which made hard cornering Golfs look like a dog lifting a leg.
Base Jetta motor is a 5 cylinder with about 150 very relaxed hp. Wait for the turbo direct injection 2 liter. The twin clutch DSG is also available.
The Audi A3, while based on the same platform as the Golf is not a replacement. Rather like Lexus & Toyota models sharing underpinnings.
Les