If you want to stay with a gas engine, I wouldnt even consier the V8.  A 'newish' V10 with a 4.30 rear is the way to go.  It will run right beside the diesel as witnessed here;

http://www.pickuptrucks.com/html/2007/shootout/hdshootout4mpg.html

Back when I was in the truck market I looked at a F350 V10 real hard, but this Duramax was too good a deal to pass up.  Your situation is similar to mine.  Using the truck only for hauling, I will never recoup the extra $$ I paid for the diesel.  Not to mention for almost two years diesel fuel cost so much more that my dollar per mile was no better than the guys hauling with gas engines.

Quote Chad.... If you want to stay with a gas engine, I wouldnt even consier the V8.  A 'newish' V10 with a 4.30 rear is the way to go.

I pull with a 3500 Chevrolet 8.1 with the Allison and 4.10 gears... My next truck, if I stay with a gas engine will have to be the Ford V10, automatic with 4.30's. The GM 8.1 is no longer offered in the 2500 and 3500's and the detuned/derated 6.0 gasser, even with a 6 speed auto, ain't going to do much with 15000#.

 I bet a newer/late model/3 valve, Ford V10, with twice the power of what you got now, will be better on fuel than that 5.8/351 you're pulling with now.

No the 3.73 would not be as powerful, and I wound not consider it.  The 4.30 is how the V10 can keep up with the diesel.  Those engines have similar horsepower but the V10 is down on torque.  However torque can be multiplied through the gears and essentially you will have the same power at the wheels/ground. 

Other options you might want to consider are: trailer towing package, camper package, four wheel drive, limited slip differential, spray in bed liner, and a factory installed spare tire.

The V-10 will use more gas than a diesel, but with a difference in cost of $6300, you can buy quite a bit of gas. Also diesel fuel is more expensive than gas, so it's improbable that you would recoup the diesel engine option cost by any fuel savings.

Gard

A camper package includes heavier front and rear springs, a sway bar and  overload springs with heavier shocks.

A towing package includes additional capacity fluid coolers, again heavier rear springs, trailer plug wiring and a frame mounted receiver hitch. Some packages include the brake controller. 

A Limited Slip Differential applies torque to both wheels, regardless of the road conditions. Without it, you could have one wheel on bare concrete, and the other on ice, and the truck would not move. It's a very handy option to have.

Gard

What about the tranny?....Doesn't it play a part?

I had a 2000 F250 diesel, a 2005 F350 diesel and now own a 2008 F350 V10.  There is no question that if you are pulling heavy and pulling often, the diesel is the better choice.  But in my case, I am not pulling the LQ too far or too often.  So far I am really happy with the V10, though I've only had it hooked to the LQ once.  It handled it fine for that short little trip.  I can say for sure, though, that the V10 has absolutely no problems pulling my GN stock trailer with two horses (6500 lbs. more or less).

All my friends also said to buy a diesel when I ordered this one, but when I asked them to pay the interest on the extra $6,000 none of them volunteered. I am kind of looking for another every day driver, but also to be a hauler if I want. Really would like the Duramax but I make way to many one mile trips. It would never get the chance to warm up and lubricate itself properly.

Definitly want 4WD, Towing package, and limited slip

Fuel mileage claims tend to be exaggerated.  The difference between the 4.30 and 3.73 would likely be less than 2 mpg empty and the 4.30 'could' get better mpg under a load.  

It will be nearly impossible to know exactly what mpg either rear end would get you in your exact situation.  And youre right, no one will agree; power or "economy".......personally I'll take power, Ive already got a car.  Only you can answer what mpg/economy is worth to you.

I use my truck only for hauling, I can save 1.3 mpg just by slowing from 65 mph to 55 mph while towing.  But after doing the math; hauling 10,000 miles a year that 1.3 mpg would save about $140 a year.  Quite literally not "worth my time".

I guess you could always split the difference and look for a 4.10...........

The ratio of the axle doesn't determine it's strength. The same axle with different ratios, will have the same weight carrying capabilities. The various suspension packages, towing, camper etc., will vary the weight carrying capabilities of differently optioned trucks.

The maximum torque values for the V-10 gas motor occur at a relatively high RPM when compared to a diesel. (3200 RPM gas- 2000 RPM diesel) By using a differential of a higher ratio, the 4.30, the engine will be running closer to its HP and torque peaks, rather than lugging at lower revolutions. This will increase its efficiency, albeit at the cost of a little extra fuel when you're running empty.

Gard

I pulled the LQ about 250 miles roundtrip to a horse sale this weekend.  I did not have any horses in it though (I was smart enough not to buy any new ones....this time anyhow).  So the trailer probably weighed somewhere between 10K and 11K.  Anyhow, this was the first decent test of the V10 engine.  I averaged about 8 MPG whereas I used to get about 11 with the 6.0 diesel.  I thought the truck did a fine job of pulling the trailer, even up some pretty decent hills between Cincinnati and Jackson, OH on Route 32.  I was going at least 60 MPH at the top of each hill.  I wasn't able to do much better with the diesels I had.  Driving the gasser requires a different driving style though.  You can't be affraid to push the skinny pedal. 

Edited to add: My truck has the 4.10 rear end.  Probably should have gone with the 4.30, but the 4.10 did fine.