I never owned anything but half ton gas trucks until I bought the 2500 I'm driving now. That was 15 years ago and I've never looked back and I've actually made the statement that "I've bought my last 1/2 ton" several times. I should have been driving 3/4 ton trucks for most of my life but never had one till I bought this one in 2002.
To me, just not that much difference between operating / maintaining a 3/4 ton diesel vs. 1/2 ton gas. And I drive a good bit. Last 3 half tons: 275k miles, 250k miles and 200k miles. This diesel: 363k miles so far. Just haven't noticed that much difference. Sure, it takes more oil and things like fuel filters and diesel now costs a little more than gas (didn't when I bought the truck), but I also get way better mpg in town with my diesel than I ever got with the gas burners. Highway mpg is almost exactly the same as my last 2 gas burners. The ability to do what I do with this truck far outweighs any small cost difference over a 1/2 ton.
Of course if you never need / use the capability of a 3/4 ton, it might make sense to go 1/2 ton. I just don't think you're gonna save all that much doing it.
2002 Chevy 2500HD Crew - Original owner - 368,000 miles so far. All stock with the exception of a lift pump and Donaldson filter