I'd recommend you drive both and see what you like. Diesels would be cheaper to run, but would cost more than a gasser with the same mileage.

Its whichever one you prefer and whatever deal pops up.

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1985 500SL Euro w/ AMG bits 130k
1984 300SD Turbodiesel 192k
1980 240D Stick China 188k
2001 CLK55 AMG 101k
2007 S600 Biturbo 149k Overheated Project, IT'S ALIVE!!!