Buy the 6.0, find a low mileage truck still under the powertrain warranty. I've worked on many 6.0l diesels and they definately have their problems. A few tips:
DO NOT install a chip or tuner, this will definately lead to head gasket failure among other problems.
The EGR cooler delete is one idea, but the root cause that I have found is the oil cooler plugs up causing an overtemp condition resulting in EGR cooler failure. So when you buy the truck, have the oil cooler replaced.
As for head gaskets, 90% of the time the gasketrs aren't bad and most shops(including dealers) sell them to save there own ass. Most of the time it is just a faulty EGR cooler. Also, the gaskets aren't the failure, the head bolts actually stretch causing the head to lift.
Leave the truck as stock as possible, I've owned my fare share of Ford diesels and in stock form they are great trucks designed for a purpose. As soon as you begin to modify them you will have nothing but problems. I am talking from experience!
I know, I know... Stock is boring, right? Trust me, I loved having the lifted truck that smoked like a chimney out of th 5" exhaust when throttled up, but if you want something dependable... LEAVE IT ALONE.