The 1.8T beetle is also 150bhp from what I can find on specs.
The diesel fuel economy against the petrol fuel economy is obviously going to depend on how its driven, but the petrol is going to have to be driven very carfully to compete with the diesel. And if you drive both pretty briskly, the diesel is still going to be better anyway.
I know what you mean about the depreciation, finding a decent low miler within price budget is a mission!