IMO the 6 speed being an overdrive transmission is actually a 5 speed where towing is considered. And it would depend on the ratios of the 4'th gear of the 6 speed compared to the 3'rd gear of the 4 speed as to which one is better for towing. This is where the downshift/upshift work is done in towing.
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2016 36FBTS Pinnacle
2016 F350, 6.7, 4x4, DRW, long bed
B & W Companion 5th wheel Hitch