I'd shoot for the diesel, one becuase of durability, two the CE's seem to rare (as you've observed Snibble), and usually demand a higher price. I forget which engine the 280 had, but it's not the most durable from what I remember...that is purely anectdotal however, since I only remember it being mentioned in passing...
I 300CD is turbo-diesel coupe (indicating a sportier nature) - quite a rarity if only in concept and not in collectors eyes.
I'd live to find a nice 300CD Turbo - manual if they made it. That would be one sweet car!