I think it depends a lot on your subject and I agree with @JanTissler. You should test both ways. I teach Photoshop on Youtube and at first I tried to talk along screencasting what I do in Photoshop. Then it seemed impossible because thinking what to say+ doing what you supposed to do on screen+ planning next thing was awfully hard. Then I wrote a script and screencast Photoshop seperately but that was not a good idea for me either because production time was too long. Writing script, screencasting, records and editing them was like a lifetime. So, my final production method formed by time and experiment. I'm now just better at talking when doing things on screen, and I can plan my video along screencasting(For example, "at this point, I should add a video (B-roll)", "If I say this, they learn it better." etc.). My first video (7 minutes) took 10 days from start to publish. Now if I have an idea, it just take 3-4 hours before I publish. So I say, just keep creating and let your experience decide.
... View more