[combatentropy]

I would say never use either. Strive to be plain. By plain, I mean both clear and unpretentious. This kind of writing doesn't have to sound informal.

This advice is not my own. It is ancient, and you could say it is the gist of books like The King's English by the brothers Fowler (1906), The Elements of Style by Strunk and White (1959), On Writing Well by William Zinsser (1976), and The Sense of Style by Steven Pinker (2014).

Bureaucratic language to me means wordiness, vagueness, and indirection. Conversational is often wordy too and likewise short on substance. But while bureaucratic sounds like it came from a robot, conversational puts the writer too much in the spotlight.

The reader comes to a piece of writing to learn something. The purpose of writing is to help the reader, as quickly and clearly as possible. The techniques in the books I mentioned, or on plainlanguage.gov, are meant to help you do just that.

[Veen]

This is generally good advice, but there are occasions in which the signalling value of jargon, formality, or complexity outweighs the value of clarity. If your primary goal is to appeal to an audience that values formality, it makes sense to be formal. Clarity is important, but it doesn’t override “write for your audience”.

[hirundo]

Like Paul Graham.