"and so on" sounds more informal. "and so forth" is more formal. However, I personally prefer "etc."

I would say it's normal to say "A, B, and so on." If you want to emphasize that there are a lot of different things (maybe reasons for doing something), you can say "A, B, and so on and so forth.." but it's not very common in normal situations. Actually, I've heard some Americans saying it the other way: "..and so forth and so on." but that seems strange to me! :)

I also prefer using "etc.", but if I had to choose one or the other I'd pick "and so forth" If I'm trying to be funny I'd say "and so on and so forth" because it is kind of an exaggeration, like what I'm saying is incredibly sophisticated.