cash for clunkers got a statistically signification number of poor milage cars off the road, that also helped with our nations oil consumption.
If it was worth it to trade your guzzler for a new car people would do it on their own without incentives. But a new car uses more energy than it saves for many many years. It's only worth it when the old car dies.
Basically how far down and across the manufacturing chain do you count the energy. Most places use the second number since it's direct energy for that car. I prefer the first since it also includes things like overhead for the sales team, the engineering, building the manufacturing plant and everything else it takes to actually make a car.