There was a similar question on the 'Ask Cathe' forum recently. I tried to post a link but it wouldn't work so I cut and copied the relevant bits for you. It is an explanation of all the different strength workouts in Cathe's own words. Which one you decide to buy will largely depend on your fitness goals. Hope this helps.
"PS SERIES........This series is designed to promote mostly muscular strength. The three tapes are designed to be used as formatted. The layout of the exercises are set up to layer on muscle fatigue as you move from exercise to exercise. Thats not to say that you cannot interchange, substitute, and or tack on additional exercises here or there, but it mainly means that to get the optimal benefits from this workout, it should be used as was created. Those who are familiar with weight training and participate in regular rotation experimentation will be able to mix and match parts of this workout to assist in exercise variety, however, for someone who is not interested in doing this, they should rather do the series as provided and then use a whole other series for "shock value" when this particular rotation becomes too familiar to the body. Remember too, that once a routine becomes too familiar to the body that it doesn't mean that the workout will no longer encourage premium results again. It simply means to take a little break from it and do some other rotation before you use it again.
SLOW HEAVY SERIES.......This series is designed to promote PREMIUM muscular strength gains. This will be achieved by going through a series of exercises, performed in a uniquely slow and concentrated manner to eliminate momentum and thereby allow for maximum muscle fiber recruitment. Remember the overall effectiveness of the Slow and Heavy Series is not so much WHAT exercises you select but rather HOW you perform them. Regardless of your fitness level, if you select a weight that challenges you for all of the exercises for the duration of each rep, you will gain premium strength.
MIS.......This workout provides a combination of strength and endurance. Depending on your fitness level, you will pull more in one direction than the other. For instance, if you are new to fitness, deconditioned, or recovering from an injury, you will gain more strength because it will be quite a bit of work to your underused muscles. However, if you are currently very conditioned with weight training, you will most likely feel more endurance conditioning (yet never fully eliminating strength gains)from this workout because of the faster pace of the reps and brief rest periods between the exercises.
POWER HOUR.......Power Hour is designed to promote ALL OUT muscular endurance, good posture and functional fitness while improving your muscle tone and strengthening your core region. You will use a lighter weight than you typically would use for a traditional workout of three sets of 10 to 12 reps. To get superior results, you will need to find the most challenging weight possible that allows you to complete each rep for each exercise yet still maintain good form and alignment. To continue to receive the benefits of this muscular endurance workout, it is very important that you select a heavier weight for each exercise when your current weight selection no longer challenges you to the very last rep."