If it is going to used mainly for towing you defiantly want a diesel just like Locu and 7mopar said. I get that question from customers a lot. If your only going to tow twice a year get gas but if the trailer and truck will become one go diesel.
As to pros and cons all 3 have them. The GM has the best trans with the Allison, if going Duramax get a 04.5 and up, if going Dodge from what I know both the 5.9 and 6.7 are good, Trans's have gotten better and can be built up. Ford 6.0 can be trouble some but can be built up to bulletproof them, 6.4's run good, I have a friend with one that has EGR delete, diesel particulate filter delete, tuner and it is a runner. 6.7's run good. Ford Trans's are ok and can be built if need be.
I would go 4WD but we get all kinds of weather here, if it's only going to be used in fair weather you can get by with 2WD. When you decide on a brand visit diesel truck websites and ask the same questions. I hope this helps.
I would own one my self but I don't tow enough to justify the extra cost of maintenance and parts.
