The 6.0 does not get better fuel mileage...at least not THAT much better, reguardless of what you're told. At best, I've seen 16-18 MPG from a 6.0. The 7.3's didn't do much better stock (no chips or programmers). My boss has an '03...I drive it every now and again when delivering equipment....gets 14-16 mpg unloaded. 2003 F250 FX4 ext cab auto/Lariat. With the trailer on the back and only a total load of about 6500 lbs, it gets around 10-12 mpg no matter how it's driven, and don't get in a hurry going anywhere. It's slow, stinks, rattles like a bunch of marbles in a tin can, but I will admit...it is kinda neat sitting next to a soccer mom and watching her rub her eyes and hold her nose because of the stench of the diesel fumes. BTW, it's a 7.3 not a 6.0.
If you have to drive it everyday back & forth to work unloaded, it'll get old fast (the diesel). The 6.0 has a LOT less turbo lag but it is still very noticeable and compared to a comparable V10 (especially the 3V) it just feels very sluggish. Don't get in a hurry. For me, it would take a LONG time to make up the difference in fuel mileage to justify spending the extra $$$, not to mention the maintenance costs. As a side note, last Sunday at the drag races, a '05 F250 reg cab 4x4 with a 6.0 PSD got SMOKED by a '94 Ranger with a 3.0 V6/5 speed. I'm sure the PSD would out-tow the Ranger by a long shot, but for everyday driving, it would get really old really quick. I think the PSD ran a 13 second....1/8 mile....at something like 48 mph. BUT, add a chip/programmer and you can drastically improve the acceleration and towing performance....but you also decrease reliability. Ford designed that engine to have a certain amount of power & torque RELIABLY, beyond that and you're on borrowed time. Kind of like modding a 2G Lightning....they hold up well if left stock; add pullies, chips, CAI's, nitrous....and you had better have another short block in the shop ready to drop in.
If you're looking at a used pickup, be sure to take into consideration maintenance and repair costs. Keep in mind that the newer diesels can cost thousands to have a seemingly minor repair done, like an injector replacement or HPO pump. If I was needing something to pull 10,000 lbs, I would look into a PSD...but it'd have to be NEW and have warranty. Especially with the 6.0L.
Im anxiously awaiting the new 6.4 (I believe thats what it is) PSD diesel to be unvailed. Should be interesting. Just my $.02, FWIW (not much these days)