That's a good question. I can't say I have a defined system and I definitely have overlap, but I try to do a lot of my in-depth tutorials on YouTube.
For example, with the YouTube/G+ Integration, I felt that would have been a long, long, long blog post and it was just easier to show via video and illustrate all the changes. Same thing with my SEO video last year. So if I feel I can make a complex topic more simple through video, I'll go with video.
I would say it definitely depends on the topic for me. And sometimes I just want to talk and I feel like I can express myself better with video so I just sit down and start recording. Sometimes it's just easier to press record and I definitely think it depends on my mood, topic, etc. lol