Honestly it really doesn't matter if it's mp3 or some video format, as long as there's *some* way to download and play it without internet. It's really nice to not be screwing with youtube on my phone while driving when mp3s are so much easier, don't lag, and don't use up data on my phone's wireless plan.
I've looked at tools that allow you to download youtube videos, but they look pretty malware ridden.