Hi CB. Definitely, absolutely diesel.
I tow a 14K 5'r with a 2012 F350 Diesel. Previously towed same trailer with 2006 F350 with Gasser V10 30V, 6.8L. I doubt a new 6.2L gasser pulls better than the V10. The diesel is so much better, than my V10. Certainly worth the extra $8 Grand.
Since buying the diesel I have towed for a bit over 10K. Up and down the east coast a few times. On flats (in Florida) and hills of NH, VA., MA. The diesel is sooooo much better.
He will not be happy with a gasser.