Given the option ($$$ notwithstanding), to me the choice is obvious, go diesel. I'm not sure it matters if it's a Ford/Chevy/Dodge (although I've heard to stay away from Dodges), but I think having the disel is key, if you're going to be towing anything big. I towed my 10k GVW 24' Haulmark w/ my GMC K1500 w/a TBI 350. Did it work? Yes. Do I think it would have worked long-term? No.
I'm not sure of the gas mileage on the Ford V10 gasser, but I have a friend w/ a K2500 HD GMC w/ their 6.0 V8 motor, and he hates the mileage, and he doesn't tow anything. A couple of years ago, I went to the June Sprints w/ John Weisberg. We towed his EP 2nd gen RX7 in his 24' trailer behind a 2500 2WD Suburban w/ a 454 Vortec motor. Towed great, but we never got over 7 mpg w/ it!!!
Everybody I know that tows w/ an oil burner says they don't even know the trailer is back there, and they're all getting double-digit mpg while towing. I can vouch for this, as I shared some of the driving to Moroso a few years back. F350 crew-cab dually w/ an early PS disel pulling a 44' tri-axle 5th. Ran down the highway like a dream. IIRC, we got ~10 mpg running at 75-80 mph, and were able to get ~12 mpg if we'd run at 60 - 65 mph.
The other thing I'm pretty firm about, is that if I'm going to have a truck, it will be 4WD. I know it costs more, and will eat at the mpg, but you'll wish you had it the first time you get stuck in some muddy paddock.
I saw a rig for sale in the classifieds. F250 PS w/ a 24' trailer. The guy was from Albany, NY, and the truck was a 2WD. Unless you only use the truck for towing, (I WISH I could be able to afford to do that), I just can't see getting something that you can use other times.
My pick right now would be an F250 extra cab w/ an 8' bed, 4WD, and a PS