Would probably be possible to compare VWs, perhaps? Especially since they sell both diesel and gas models in both Europe and the US (though not sure that all the engines are the same).
As to the grass is greener thing....I think it's fair to say no matter what the the fuel economy in a diesel is going to be better than that on a comparable-performing gas car - though that doesn't take into account things like car balance (for handling), price, accelleration, etc which are "other factors" that would outweigh fuel savings to many people. Also doesn't take into effect actual emissions - but let's face it, very few people buying cars really care about emissions if we're honest with ourselves ;)
Actually a good example would be the current US-market VW Golf Diesel vs. Golf TSI vs Golf PZEV. Again, not straight comparisons, but the diesels are roughly 10-20% better fuel economy with reasonably similar power/tq numbers. IDK.
Diesel rated at 140hp/235tq, 31/42mpg TSI rated at 170hp/200tq, 26/37mpg PZEV rated at 170hp/177tq, 24/31mpg