Neither of us huge fans of either technique as a stand-alone teaching tool, but we brainstormed a few ideas for testing which method is more effective.
Pick students at roughly the same level (determined by some sort of assessment) and give them either live or recorded video on a new subject, then assess what they learned. These assessments would test both basic conceptual knowledge and the ability to apply the new skills to alternate situations. Some ideas for this assessment would be to have them complete a project using this new skill or teach another student this skill.
Have students watch both live and recorded lessons and interview them to see which they preferred. This may not get at how much they learned, but it will indicate which they objectively prefer.
An eye tracking study to see where students are putting their visual attention. While this method won't give an idea of where they are directing their auditory attention or the amount of attention they are paying it will give an idea of where they primarily look.