The Prius will be no better environmentally if your going to do a lot of motorway miles, the hybrid motor will only kick in around town, therefore your lugging around a lot of excess weight on the motorway. Father in law had a 2lt diesel Avensis, averaged around 55mpg on the motorway.
There was a feature in a recent edition of EVO mag, comparing the Prius against the new 100HP Panda, on mixed driving conditions the Prius was slightly better, but had a much higher price, so over a 3 three year period the Panda was cheaper to run.
The "greenest" thing you can do is not buy a new car.
