I also think it depends on where you're working out, and what kind of mood you're in. If you got something heavy on your mind, I would say cardio with no music. It'll give your brain a chance to sort things out. Than again, you may need a little bit of motivation, some some fast paced energetic music will get you through the run. I wrote a bit about this in my post
Too Busy To Exercise, and how I visualize during my runs.