if you want economy go for the diesel , yes it will cost more to buy but if the price difference isn't an issue that is what to go for, ive got an 11 year old 1.9 diesel volvo that has the same engine as the laguna
volvo detuned it a little to 115bhp opposed to the 130bhp in the laguna and it currently averages 42 mpg in the winter , in the summer it rises to 47/48 average which is excellent for such a large car and engine