If you buy a single rear wheel, your only choice is 3.42.
If you get a 3500 dual rear wheel, you can get 3.42, 3.73, or 4.10. The GCWR and max tow rating increases as you move from the numerically lower axle ratio (3.42) to the higher ones (4.10).
Also, if you choose the 3500, you can get three versions of the 6.7L Cummins:
350hp/660tq with the 6 speed manual
370hp/800tq with the RAM 68RFE automatic
385hp/850tq with the AS69RC Aisin automatic
Plenty of options, and the tow rating varies from 17,xxx to 30,xxx depending on cab, engine version, and axle ratio.
You have to decide what you want.