I prefer long videos, since I usually draw while I'm watching, so having to constantly select a new video and hit play can be irritating. The real clincher for me, though, is if you're narrating on the track or not. I'd much rather hear your thoughts on what you're doing than a listen to a few songs or, even worse, silence. A two minute video with your narration would be worth 10 twenty-minute videos with a random soundtrack.