Lower mileage is exactly what it says on the tin, its less miles on the engine etc. But if someone has ragged it for 30k miles vs someone who has done all the servicing correctly and driven it properly with 60k miles, you are pretty sure that the 60k miles car will run better and for longer than the other.
That said, for some reason the UK market is a fickle one, for some reason we are obsessed with mileage (I lived in France for a while no one cared a jot about mileage) so lower mileage cars get a premium, even if they are dogs. 60k miles on a V6 GT car is nothing really, most of these will hit 100k and keep going (again the UK market seems to believe there is some hex that hits cars as soon as they hit 100k miles) - if you have no intention to sell this car soon, then go for condition, if you want to sell it on within a couple of years a low mileage car will help you sell in this market.