That depends on the year. Up until 1974 or so they only came in 100, 200 or 300. Then the 150 became the "heavy half" with heavier springs and engine options.I was told that the 100's have lighter springs in comparison to the 150's too. But that's the only difference that I have heard about.
For example, they built W100 and W150 Shortbox pickups. Just the spring rates were different.
About 1980 Dodge switched to the 150, 250 and 350 designations, and put the 100 as a "light duty" truck with reduced GVWR and payload capacities.