If I was a betting man I would probably go for the petrol, environmental concerns will probably mean in a few years the government will start penalising diesel powered cars more and more and will probably lower the limits on the annual MOT test to the point that it will be almost impossible to get them through with out spending a serious amount of cash.
On a pure investment point of view it will be another 5 or 10 years before they are looked on as a true classic and who knows what will be happening by then with internal combustion powered cars. I suspect battery power and/or hybrids will becoming more common place and potentially a lot cheaper to run. It will be another 25 years before these cars start getting into the £0 annual tax bracket and I don't think there will be many of them left by then and if the government start charging us £1k a year to tax them that will soon see a few getting scrapped probably due to high annual running costs.
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