Right now I have a Ranger, a 1/2 ton van, and dad has a 3/4 ton Dmax and a 1-ton Dmax. In the past I have had a few Powerstroke 7.3s, a 6.5TD, and a 12v Cummins as well as a ton of gas buggies.
I'm biased. For both off and on road stuff, I prefer the compact and 1/2 ton, but I don't have a family full of kids. The number of times per week I have a single additional passenger in my vehicles is maybe 1 or 2. Having said that, I am a huge diesel lover. I spent months trying to find a decent and applicable diesel to swap into the ranger, and I'm even swapping a Duramax into a 66 Bonneville. 3/4 and 1-ton trucks (in my opinion) just tend to suck off road. Stiff, non-compliant suspensions and punishing ride.
My gas Ranger gets 12/16 mpg
1/2 ton van gets 10/15 mpg
3/4 ton Dmax averages 22 mpg, mostly highway, with a ton of mods (938 rwtq)
1-ton Dmax dually gets 18-19
You can't really base the MPG benefit of a diesel as part of recouping your initial costs. It take a LOT of miles to recover the money you spend on the fuel you save. What you can bank on is resale value. Buy a gently used gas truck for $10k, drive it for 100k miles, and you have a $2500 truck. Buy a gently used diesel truck for $20k, drive it for 100k, and you have a $17k truck. If you sell it, you have way more money in your pocket with a diesel. If you go that route, be aware that some diesels are a good bet and some can be a money pit. 6.0L powerstroke that hasn't already been "bulletproofed" by someone else means you might have to drop $4500 some day to fix it. 6.7L stroke, good bet. Duramax after 2004/5, good bet.
Another thing to note. Post-08 diesels with a DPF seem to have a significant "break in" period during which the DPF causes disappointing MPG numbers. Dad's 1-ton is the only DPF truck we have, and it started life getting 13 mpg on the highway. It was only after about 25k miles that it started getting better.
The only other thing I'll mention is beef. This is very nebulous and very dependent on the year/make, but stepping up to a 3/4 usually nets you WAY beefier stuff; ball joints, axles, brakes, cooling systems, transmissions and TCs.
I guess what I'm saying is, pick what's important to you. I would think a 1/2 ton would be enough, especially with the beefy stuff they're putting out these days.