Hi, Jeanne
Having done all these workouts fairly recently, I hope I can help.
Step Blast is so fun, and I believe, somewhat more intense and complicated to learn than SJP. The combinations are longer and more involved, and then they are linked together at the end with some interval blasts that will really promote cardio improvements. The fun factor is definitely comparable to SJP.
(You should know that I love all of Cathe's step routines, don't care at all about music--the cameraman could be tapping his foot and I would be happy--and am used to modifying up or down depending on what state of revolt my body is in on any given day.)
I have Powerstrike Millenium II and have done it many times. It is very fun and intense with good instruction. In those ways it is like KPC, but it is different in that it is team taught and has much less impact than KPC. They work one side for about twenty minutes before switching to the other side. (When I first had this tape I would only get sore on the side I worked first because I'd start to run out of gas for the second side. You can even this out easily enough, though, if it happens to you.) I recommend it highly.
I could make lots of recommendations, but I'm kind of a cardio omnivore; what else do you like? Have fun with your new workouts!
--Ann