A summary of research and anecdote on writing with a pencil versus writing on a keyboard.
In brief, pencils are good for taking notes when you want to understand or remember something, and also good for doodling which correlates with creativity.
Keyboards are good when the time comes to write something for an audience. Particularly fast typing, a minimum of 24 WPM, but the faster the better so that you have transcription fluency: you can get the ideas in your head down on the screen as seamlessly as possible.
An experiment has been done on High School students in Australia on transcription fluency and touch typing.