Ill add my .02, all pertaining to my experience with Diesels. Ill ramble off topic but hey, why not? Every thing I mention here in no way pertains the the Raptor but does give you a biased point of view on some of your other options.
I daily drove my 2001 Cummins truck for 8 months with my loaded (1000+ lbs) 1980 something FWC Fleet 32 miles round trip to work from July to February (summer and winter diesel). My drive is a mix of city during the day and hwy at night. I averaged no less than 17.5 MPG hand calculated for the time the FWC was on it. Now that my FWC is currently off, im sitting at about 20mpg and I will continue to keep track of what I get.
My truck is a 01 quad cab short bed high output Cummins with a 6spd manual, 3.54 gears, stock sized AT tires, Power Puck, 4" MBRP exhaust, and I run 1 ounce of TCW3 two stroke oil to every gallon of fuel. I do not exceed 2k rpms for while cruising but frequently make black clouds. I had an 03 reduced output (cali truck) with an automatic and a smarty jr on it that would get 15mpg at best in town but I saw as high as 24mpg unloaded on the hwy. My Dad has an 06' Cummins mega cab with an automatic that's bone stock and driven by an old guy, he never sees above 16mpg hwy and I can only imagine its equally as bad in town. We have towed cars, boats, and junk and the 5.9 Cummins never even knows its got a load back there. Im a little biased towards Cummins and Diesels in general but I feel that I have good reason, plus 6cy's and one head has far less going on than any V8 ever can.
I am a mechanic for UPS and I have a fleet of 42 cars, 20 of those have International engines, both 7.3's and 6.0's. Based off what I know from working on them the 7.3's and old IDI Non turbo Powerstroke's are the only acceptable Ford trucks, in MY opinion. I have done far too many injectors, Turbos, HPOP's, etc etc etc on 6.0's and I can only assume the newer stuff is slightly better but still flawed. I think its very worth mentioning that a co worker (mechanic of 30 years) has an F150 he purchased new in 03 and claims to have not done a single thing to the engine other than a throttle body and oil changes in 270k miles.
I also worked on GM's for years and the Duramax never impressed me with anything more than its power and speed, its more like a high power gas engine, but its more sensitive to fuel contamination and doesn't get the best MPG. The GM gas engines are in my opinion simple and pretty robust, but like most gas engines in a truck the MPG is decreased over the Diesel counterpart as is the lifespan.
As far as my work experience with Cummins goes, we only have ISB 02's and 07's, so that's relative to 98-02 5.9's and 2007+ 6.7's. we reseal oil leaks, change water pumps, replace an occasional VP44, replace sensors, clean EGR's on the 6.7's, lift pumps, and we occasionally replace and engine with excessive blowby. A 6.7 Cummins currently has the record for most miles in our facility with 480k miles with no major repairs (300+ miles daily), the next closest is a MB904 diesel with 300k and basically no repairs.
I definitely need to mention that we also have a metric **** ton of cars with GM 6.0 gas engines. None of these last longer than 170k miles but its ok they cost 1/4 of what any of the diesels cost and require much less in maintenance. Please keep in mind that 98% of the UPS drivers drive these cars harder than Colin McRae drove with Subaru so this is a worst case observation.