In my experience in Europe they always advise you take the up-front tank as it's cheaper, and it's true you usually pay around 10% _less_ than most gas stations

In my experience in Europe they always advise you take the up-front tank as it's cheaper, and it's true you usually pay around 10% _less_ than most gas stations. However that's still only worth it if you manage to return the car with 10% of a tank remaining (which basically never happens)