A dually isn't required to pull a fifth wheel assuming you don't need the pin weight capacity. I have both a DRW 3500 and a SRW 3500. I pull a 38' Fuzion and a gooseneck equipment trailer with both of them. The DRW may feel somewhat more stable but really if I couldn't run them side by side I wouldn't know the difference. You have more tires obviously with the DRW so it will be more safe if you blew a tire on the rear and that extra set of tires does help with stability but even in high winds (which in Idaho is all the time) I have never felt like the SRW caused me to alter my speed or that I was in an unsafe situation.
If I was putting that many miles on, and most was unloaded, and I didn't need the DRW for pin weight, I would go SRW as well. They are cheaper to run per mile, better in Winter driving conditions, and easier to get around town in. However if you need the capacity then you have no choice but to go DRW. Otherwise go SRW and with that many miles going Diesel can pencil out. As far as personal choice I like the Duramax, but whichever you chose will likely be fine. All three are good pickups.
