[Cthulhu_]

That's it, it'll depend entirely on the context. Maybe if you're selling something you should write like you talk, or if you're posting comments / opinions on an orange/brown/grey website, but if you're writing "A History of Ancient Britain", it's not exactly you'd talk to your friends about; it's a book, it's to educate and to entertain. Do you talk to your friends to educate and/or entertain? I mean the latter, sure, but people even change how they talk when they are entertaining someone else, so. idk.