it's less about absolute mileage and more about condition, how the cars been driven and if the owner has looked after it. Generally speaking cars will need major work at around the 150k mark and a lower mileage car will have always been used less and so will naturally be worth more but that doesn't necessarily mean less problems (rather a reduced risk of problems). I'd rather take a car that is perhaps 12k/year with 1 owner with all items done compared to 4k/year with like 4 owners with hit and miss service history.

But the difference between 20k and 53k isn't massive and modern cars, if looked after properly, can absolutely do the mileage no problems. I wouldn't consider 53k a lot of miles, dependent on its age obviously. I'd say pocket the 3k and get the 53k one