I've had two F350's and they were both 7.3 powerstrokes. They were really good trucks and real workhorses. Those big trucks became a luxury I didn't need anymore (sold my business). Also the upkeep is something you should think about when buying one of these things. For example, fifteen quarts of oil every oil change adds up. Plus if you have a problem you can't work on it yourself unless your a diesel mechanic.
DO NOT BUY A 6.0 DIESEL, you will be sorry, total pos. I don't have any experience with the new 6.4, but have you seen under the hood? Good luck. I heard a guy with the new 6.4 had a vacuum hose problem and the most effective way to fix it was to pull the body off the truck.

Now what if that happened out of warranty?
If I was to buy a new one I would get the V10 even though the gas mileage would suck. That motor is bulletproof and the Ford chassis is one of the best out there IMO. Solid truck with the V10.