Read through the entire thread and actually found a thread back when you were asking questions about the Wolf Creek 840. That old thread showed real world numbers of 3320 pounds so will use that number in the discussion.
When considering gears, consider there are lots of folks hauling large fifth wheels with SRW 3.42 geared trucks. The only complaints about them are the gearing at take-off is to low. Once up to speed they are happy. So I would think the 3.73s should be fantastic unless you want a rocket ship when loaded. Also, the 4.10 gears get you to over over a 20,000 trailer rating which most folks don't need. Even my old 5.9 with a mere 625 was rated to tow over 15,000 pounds with 3.73 gears and did fantastic with 12,000.
The new 2015 trucks haul a lot more weight. My sig truck has an available payload of 4,300 pounds. While it is a gasser, the diesel has almost exactly the same payload because they raise the GVWR from 11,300 to 12,300. My camper weighs 2,400 pounds, but I am still under by 1,500 pounds. So with your 840 and extra dogs along with you boys, you will probably be right at max for the truck. Not sure how much trailer you are towing as any hitch weight would probably be over.
In regards to the automatics behind the cummins, it was explained this way. They are both very good and odds are you will not have to repair either unless you just abuse it. However, if you get the Aisin, except for maintenance, just forget its there.
One of the advantages to your current 2007 is it doesn't have DEF. Plus the newer exhaust brake is also more complex. Then there is also more complex computers, almost all switches are either on the computers USB bus or radio computer. And even those two computer interface to each other.
Last point is why has your 2007 truck had a sudden increase in value? Could it possibly be concerns with the latest trucks and their DEF? Heard some place, but can't confirm, that the DEF system is usually only good for 100-150k before needs a total replacement at a cost of about $6k. So maybe the newer trucks won't hold value as well as the older ones.
That said, I moved up from a 2004 manual CTD SRW to a 2015 auto Gasser SRW and have been very happy with the mileage, power (for my needs) and build quality. Some things I don't like such as the electronic shift 4wd, the electronic shift control of the automatic, and a couple other minor things, but nothing that would make me hesitate from buying again if something happened to this one.
As goingthisaway said, our father just ordered a 3.73 Aisin 4wd long bed dually to haul his Bigfoot 3000 TC or haul is 14k fifth wheel. When helping spec out the truk, he too felt the 4.10s would be too much. Figured since he was happy with the 5.9 and 3.73s, the extra torque of the new engine would off set the higher transmission ration