If I were to buy a new truck today it would be a 3500 SRW Aisin QC LB, and it would have dealer installed 4.10's in it before I ever drove it. They would be part of the original purchase, and Dodge installed. That should at least get me a 12/12,000 warranty on them, if not the full warranty.
In terms of wear and tear, yes wear and tear on the engine is lower with 3.42's when there isn't a load. But if there is a load the engine has to produce more torque to match the rear wheel torque of the 4.10 truck. Torque is harder on parts than horsepower, so when loaded the 4.10's are actually easier on everything. (not to side track, but I just did the math for a similar discussion... a truck with 3.42's needs to make 118hp and 292 ft/lbs to get 1,000 ft/lbs to the ground and a truck with 4.10's needs to make 118hp/ 244 ft/lbs to get the same 1,000 ft/lbs to the ground, this is all in direct gear at the same speed).
The gearing of the Aisin makes 3.42's work well for nearly everyone, as most people spend far more time at highway speeds with a load than at slow speeds which means dropping a gear is easy. The reason I want 4.10's is the amount of time I spend at low speeds, sometimes for 60-70 miles without getting above 40 mph and 1/2 of that below 20 mph, with some steep grades and GCW's at or above 20K lbs. There isn't anything lower than first, and if I am in first working the fool out of the torque converter there is going to be a LOT of heat generation. This is the #1 reason I have a manual in my 2005. 4.10's provide 20% more gearing, and that will be noticed when loaded at 7 mph on a 10% grade at 7K feet.
Attention: TDR Forum Junkies 
