I have 4.44 gearing with my Aisin and CTD. It does great at 65 mph or less, but I would not suggest this ratio for people that drive faster. 4.10 gearing will work well for those towing 65-75 mph. If you drive empty often, the 3.73 ratio would be my choice. Any higher gearing will limit you on take off (stressing the transmission) and most likely require 6th gear be locked out when towing. Ram offers a 4.88 ratio but this is better for local local delivery setups than cross country cruises.
If you choose gearing that is too low, you can go to taller tires later to make some adjustments. I currently run 225/70r19.5 tires and would not hesitate going to 265/70r19.5 if I need more clearance or a wider footprint.