Writing is better than speaking because the individual letters can be directed in their flow much more precisely than words rushing out of our mouths. Speech is more autonomic, less conscious. It must originate from within our reptilian brain. Writing is very much a conscious activity. It allows us to be more ourself.
E-mail is in between. Its writing that can flow more like speech because we can use all those shortcuts, such as LOL and leaving out punctuation etc. So we need to me a bit more careful too.