3.21 rears on VHD dump truck

I am going with the overdrive IShift transmission. I am going to upload the information for the IShift and the Eaton 18 speed with the transmission ratios. I don't see that much difference between them to warrant the different gearing for the 3.21 rears and the 4.10 rears. The salesman says that the majority of people that get the VHDs for dump truck work around here get the 3.21s. I think I should be in the high threes at least for getting unstuck in soft sites. But I still want tall enough gears for the highways. I am going to have this truck for at least 5 years if not 10 or more. The correct ratios will make or break the functional ability of the truck. I think the people ordering the 3.21 rears are relying on the higher torque at low rpm of the D13 engine but not having any experience with it I don't know if it is enough. The truck is going to have 22.5 tires. Much thanks to those that already replied specially KVB and Uncleal13!