The real benefit of using the 600 for both main and background light is the ability to use the UNIT/ALT feature to do masking.
David, here's my question. I'm a fan of the traditional method well demonstrated by fjhphotography. So I'm wondering: While the masking function works well with Manny in your demo, will it work as well with people, especially with groups of people?
Related to the above, how closely together do the frames need to be fired in order to eliminate normal body motion so the masking frames will actually work together?