[MandK]

The 3.55 will get better mileage towing than the 3.31’s, and the 3.31’s will get better mileage running hwy speeds not towing. The difference isn’t much, like 1 mpg or less.
Either is fine for pulling, there’s plenty of engine and tranny to make up the difference.
If I were ordering a truck for full timing or towing a lot I’d go 3.55. Not towing a lot, 3.31.
Buying off the lot…doesn’t matter imo.
We bought ours in early ‘21 when ordering was taking a year and no dealer within 300 miles had 5th wheel/gooseneck packages so we went out to the mid west truck shopping. All the dealers that had trucks on the lot prepped for towing had the 3.55’s in them so that’s what we got.