When buying anything used, a lot depends on the vehicles past maintenance history..
In my case, I have a 13 F150 Eco bought brand new, so I know my own maintenance history, but do I have proof of all the fluid changes and other minor stuff I have done on my own? Not really. I have a written 'log' of it all, but no receipts or other junk like that.. I have the recall paperwork done at the dealer and such of course.
Anyway, I've got 116,000 miles on my F150, but have not had any turbo related issues as such. Or timing chain/phaser issues or really any of the other stuff you will read about on the internet.
The 5.0 is not without it's 'issues' either...
On my rig anyway, "turbo lag" is not a thing.. I've never experienced it. Stab the go pedal and you GO...
The 5.0 needs rpms to get into it's power band, so high revs is what it likes. The Eco gets a lot of it's power down low torque wise.
140,000 + miles on either one will really need to be gone over, as that's about the time most 'major' things might start popping up in either flavor..
Again, it's oil change interval's is the biggest things.. If the previous owner only changed the oil when the dash light came on... Well... I'd walk away fast! if they changed around the 4-5,000 mile marks, then it's a lot better for sure!
Anyway, good luck with whatever you decide on!
Mitch