6.4 issues:
leaky rads and hoses,
hanging injectors causing a melted piston,
cracked pistons for no obvious reason,
failing belt tensioners,
rocker arms wear out and/or break prematurely,
egr coolers crack, causing bent rod or hydrolock scenarios,
front covers corrode/cavitate which causes coolant in the oil,
hpfp harness chafe on job 1 models,
excessive regens cause diesel in the oil which can increase bearing fatigue,
poor maintenance and high mileage is the killer on these engines.
i had one 6.4 that had 170k miles on it, not one single issue except the hpfp at 85k. it was maintained impeccably and was tuned and deleted when it was a year old. there are others that have had much worse luck than that though.
if i were you, i would look for trucks in the classified section on this site that already have a lot of mods done to them. you save a pack of money and likely the issues have been addressed.
As for the 6.7, tuning is making rapid advances and if you are talking stock to stock, the 6.7 blows the 6.4 away.
you need to make a decision on what your goals are for the truck. if you want a reliable daily driver that has enough power, go with a 6.7. if you want a performance machine that has enormus power potential, go with a 6.4.
the 6.7s have warranty and are a more comfortable truck, also the aftermarket support will only be getting bigger.
