I posted this on another site as well. I have decided that the F150 I have is just not going to cut it with the trailer I have. Have an opportunity to get a pretty good deal on some 2015 350 Super Du...
If you can afford it, I'd go diesel. Of course this depends on the weight of your current trailer. But remember, you may want to go bigger and heavier in the future.
I just got a diesel in May, towed a 10,000 pound 5th wheel through BC to Kelowna this summer. It was a very worry free ride. My gas one would have made it, but would have been switching gears constantly. As long as I am pulling a trailer now, it will be a diesel.