Which diesel get's that much loaded? I know that my Dad's Duramax gets about 12-15 with his 5th wheel, my buddy's v-10 ford get's about 10 pulling a full load of hay bales. Ford powermax info i've read is maybe a little less than the duramax, but sometimes i read people getting up to 15.
So, if those numbers are a little more accurate, then I'd say you should be comparing 10mpg gas v 15mpg diesel Let's also say you change oil every 10k miles (easier for math). Lets say you are paying 5bucks a quart for something nice. Let's use US averages on 8/2/2010 for fuel prices.
100,000miles on gas would be 10,000gallons of gas ~6quartsx10 changes = 10,000x2.735 + 60x5 = 27,350+300=$27,650
100,000miles on diesel would be 6,667gal of diesel ~15quartsx10 changes = 6,667x2.928 + 150x5 = 19520 + 750 = $20,270
7,000 more to run a gasser v10 over a diesel. and that's if you were towing for 100,000 miles. i'd rather pay that 7000 over 7 yrs than the difference in cost when new. I wish it were as simple as that math, though. resale of diesel will be higher, likely. and then there are things like fuel filters and spark plugs and injectors and things that are pluses on one and not the other.
my opinion is that it really doesn't matter which you get. Heck, I'll drive a ford v8 3/4 ton. I don't need to move the wood fast, i just need to move it.