For basic out of the box reliability, a 7.3 is probably the way to go. Well maintained, they can last for quite a long time. The tranny is quite often a weak spot in the 7.3's.....
For more stock power, a 6.0 is the way to go. They are more fussy, and some do have issues. They require a bit more monitoring, and are sometimes less reliable as a whole than a 7.3. Common upgrades to improve reliability include oil cooler upgrades, head studs, and egr deletes. If you solve some of the basic issues, commonly known as "bulletproofing", the 6.0 is a a good engine. And the trranny is tough as nails.
Overall, if you want a truck you can just drive, I would lean towards a 7.3. If you want a a truck with more power out of the box an ad don't mind some more meticulous maintenance, go with a 6.0