you shouldn't get a 6.4 unless you're prepared to put $12k under the hood at a moment's notice.
you likely won't have to. I've owned a number of them. Currently still have one and would drive it anywhere. I've never had a problem with any of my trucks. But I've seen enough chit go down to know what COULD be around the corner.
They're my favorite trucks, so I live with that knowledge and I can afford a new engine if necessary.
Neither of those trucks would be my recommendation for an intro to 6.4 ownership unless that CCSB is still owned/driven by the original owner and they have maint records.
I wouldn't touch the F450 w/ a 10ft pole, personally.
You need to look for a crystal ball that will tell you when/if it's going to crack a piston and what kind of oil change:regen ratio it experienced so that you can make an educated dice roll on what kind of health/lifespan the needle bearings in the lifters have left.
have fun!