But why do you choose one or the other language?

Preference.

If you want to learn the alphabet, it doesn't matter what letter you start with.

The main reason for using javascript would be that it runs client-side in a browser and almost everybody in the world has a browser. I am using Python because it is usually shorter and simpler, but the underlying concepts are the same across all languages, and especially here; The STEEM blockchain has a very limited set of operations and calls. With Python it is easier to read the source code (later chapter) to understand how the 'raw' API calls work.