Inspired by my own answer here, I'm wondering what the actual numbers are regarding gasoline and Diesel fuel refining. I have read (don't remember where) that refining of Diesel fuel is less environmentally damaging than that of gasoline. But is this actually true?

And for the sake of this question, I'm interested in the environmental impact of refining per energy output (not per volume of fuel), so the energy density of the output fuel ought to be considered.