I am a huge fan of the Cummins (own three currently)... have owned 20-25 in my life. I buy the older 12 valves cheap and use for a while... post for resale at 150%-200% of what I paid and resell when a buyer comes along (buyer ALWAYS come along sooner or later).
The two facts I can state about a Cummins motor truck...
1) It is the ONLY truck you can buy and use every single day (use in a "normal" manner (i.e. not abuse like an 18 year old)). And it will go UP in value. 7.3s and 22R somewhat similar, but not like a Cummins.
2) Miles mean NOTHING.
My three currently owned have 285k, 255k and 425k
Example: Bought this 1993 in 1999 for $10k... drove her 9+/- years and sold for $10k... guy that bought her could resell for $22k+ in today's market