I think it depends on what type of condition you're in. If you're totally new to exercise, I'd do two week of SIU, and then two of RIU, and the last two of BIU.
Only you know what type of shape you're in. How are you doing with the lunges/squats, etc.
If you've been exercising for a while and Debbie's fast pace doesn't bother you, go with the first week of SIU, the next two of RIU, and the next two/three with BIU. If you have Keep It Up, that can be used in your last week, also.
Each workout is supposed to prepare you for the next weeks workouts. Start It Up begins with no weight work (just your own body weight.) Ramp It Up has the weights, and is a little longer. Burn It Up is the toughest/longest of the three. Keep It Up is used in maintanence, or as an alternative to some of the other workouts.
Hope that helps somewhat.