2005 ram 2500 4x4 quad vs. 2006 ram 1500 quad 4x4

I vote the 2500 for sure. I had the exact same trailer and and F-150. It pulled it but payload was lacking for sure. I got a 250 and it was night and day. Id have only one concern for you and that's the fuel mileage you are hoping to get. I'm not saying you are wrong but I doubt very seriously you were getting 15mpg while towing that trailer. The readout may have said that but there's no way. The 2500 will not do any better on fuel either. Just more enjoyable tow.