Overall, the 1/2 ton is definatley gonna cost you less, especially if it's your daily driver ... I used to get 17@75 mph, 19@65mph with my 2000 5.3L Chevy .... I get about 15@65, 14@75 with my 2005 8.1L ... I'd get about 14 around town with the 1/2 ton, I get 8 or 9 with the 3/4 ton
But, towing the exact same trailer, I got 9-10 mpg with the 5.3L and 9-10mpg with the 8.1L ... and it's not even funny how much better the 2500HD with that 8.1L pulls my trailer ... everything from suspension to brakes, everything .... Pulling my dad's 24' Charmac, I still get 9 mpg with that trailer too and it's bigger and heavier than my 16' Mirage.
My 1/2 ton pulled my little enclosed well enough, but it had 150,000 miles on it and was costing me more to repair on average every month the last 6 months I owned it than the payment on the 05' 3/4 ton I bought is ... The main reason I went with the 3/4 ton is I posted some questions on a few various sites, and I couldn't tell that there was any appreciable difference in fuel economy on the highway or towing between the 6.0L and the 8.1L motor, hence my decision to try and find an 8.1L .... My dad's 2008 6.0L 1/2 ton pulls his 24' Charmac just fine, pretty well actually ... but towing, he only gets about 10 mpg with it .... So I kinda figured hell, I have a truck to primarily haul my toys, even though it's also my daily driver ... if I'm not really saving that much more on gas buying the 1/2 ton or 3/4 ton with the 6.0L - at least when I'm towing - I might as well just get the 8.1L ... I have a company truck I drive for work and don't pay for gas on, so mostly during the week I drive that, then haul toys on the weekends, so that also played a role in my decision to get the bigger more expensive 3/4 ton ...
Sucks on trips that are unloaded, but I sure do like it pulling the trailer ... but, I'm also paying a price for it when I'm not towing anything, that much is certain. There's pros and cos to both .... if it's worth it to ya to feel more comfy towing I guess buy the bigger truck, but it's gonna cost you probably on average a few hundred more a month than the smaller one will.