AFAIK, there has been no scientific testing back to back of a Prius and a TDI. The anecdotal evidence is the TDI routinely returns 45-50mpg even cruising at 80mph. The Prius web boards have owners posting mpg's far below the EPA figures and well below expectations.
I have driven the Prius. My neighbor has one. Decent car, typical Toyota build quality. To me it feels lighter and "cheaper" than my TDI but that it subjective. Seat of the pants says its not as quick either. I also don't care for the styling. If I was buying a car strictly for stop-n-go not over 45mph commuting then the Prius is excellent. On the highway the TDI is a far better choice.
The scariest part of hybrids is that Toyota has refused to say how much they actually cost. The implication here is that they are building, or have been building them at a loss. The idea is to dump the vehicles and get consumers hooked on the technology. They make their money in the future when the technology is accepted and they can start charging what they want.
Look at the latest hybrid advertising campaigns. The Honda Accord and Lexus SUV hybrids are only hybrids to accentuate the performance while doing very little to improve the mpg's. They are trying to sell the cache of hybrids at the top end of the market. Yeah, great. How about focusing on economy, actually using the hybrid system to do what it was designed for. Diesel will likely never be misunderstood as a performance vehicle. They may be capable of it but likely will always be associated with thrift. RT