While the Plug-in-Prius gets
While the Plug-in-Prius gets better gas mileage when you deplete the battery, that totally misses the point. The Volt gets way higher average mileage as owners are showing.
The crossover point for when a journey becomes more efficient in a Plug-in-Prius compared to a Volt is 97 miles. So if you are driving over 97 miles every time you get behind the wheel, buy the Prius, otherwise the Volt is the better bet.
The Plug-in-Prius claims to go 13 miles on electric power but the reality is that for anything more than light acceleration, the engine still comes on. That is not the case in the Volt: You can drive up to 50 miles gas free. Government stats from both the US and UK show that 98% of journeys are under 50 miles. For the other 2% you still have the backup gas engine.