I was thinking about this just yesterday and, under my new rules, both cars are worth the same as both are "underused". They've sat about barely being used but, and this is the important bit, they've had equal chances of scratches, scrapes, car park dings and something failing prematurely because they've never had a full shake down.
"Low mileage" and some boost to along price means under 10k miles. Or 10k kilometres. Or something stupidly low. Something where the owner left it in a garage and either forgot about it or thought it might make him money because he's a massive cock. The actual few cars that really haven't had a chance to rot.
I sold my mother's Fiesta last year. 53 plate/22 years old, 1 owner, 30-odd thousand miles. The door trim had still fallen off (and been stuck back on), the steering wheel was peeling, there was a mildly rusty ding, boot release was flaky. It wasn't bad by any stretch but, it wasn't factory fresh which is what people seem to think "low mileage" means. The one thing that would price it above anything else was that it had been garaged from new. And that's something that is almost impossible to guarantee on the majority of cars unless they are a one owner job. And then it's irrelevant whether the mileage is 30, 80 or 180k miles as the driving is only 6000 hours of use compared to the 125,000 hours of sitting out in the elements, quietly suffering.