Since you don't tow a heavy load all the time ("two to four times max."), I would save my money and get the 1500 w/ 3.9 gears and 17 inch tires. You can always add something like this http://www.suspensionconnection.com/...-air-bags.html
if you need a little more towing capacity. This was just a quick search, I'm sure there are better kits out there.