Is it the miles or the age of a well maintaned truck?

There are other variables to consider, like price, has the vehicle been well maintained? I have never been able to convince myself that a used vehicle with over a hundred K miles was a good buy. Others may have a different opinion?

The 50k truck by all means. Six years is not long enough for a lot of age related issues.

It depends how those miles were acumulated if it was mostly freeway and care of vehicle and body but i would still go with the 50k

Depends on the upkeep by the owner. How does it run? Does it need brakes? You can check that visually. Ask the owner what service has been performed. Let him answer. If he doesn't indicate the tranny was serviced and it has 150K miles on it, I'd walk. How do the tires look? Crazy wear on the tires indicates lot of suspension work required. Take it to a place like Autozone and have the codes red. If not removed properly, they stay there. etc.