For me it was all round costs - so mpg, tax, list price and residuals. Was about £80 difference a month for me over 4 years (if doing 15,000 miles and using 35 and 45 as average mpg's.)
I've always enjoyed fast diesels though, so doesn't feel like a compromise to me at least - although I'm sure I might have got the bug had I test drove the GTI.
