I don't know if I'd go with the 3.73 if you only tow a few times a year. Don't get me wrong the lower gearing is awesome, but it will impact your MPG. 3.55 seems to be a good compromise.
Also think if you plan to change tire size down the road. If that's something you normally would do to a pickup, then the 3.73 is a good choice as it will help compensate for a larger tire.
2015 F350 LARIET CCSB SRW PSD
2017 KEYSTONE FUZION 325