Power Hour is a full body strength workout that is designed to increase your endurance. As Cathe says in the beginning of the video, she calls it a "high rep, muscular endurance workout." Higher endurance is good for being able to hold heavy things for a longer period of time and also good for toning I've found. (Another Cathe quote "It's all about how you spend you time under pressure" - as you sweat through the low ends!) PS on the other hand I believe is focused more on building muscle, actually getting stronger (although I've found that PH has improved my performance with PS and vice versa).
In PH you would need to use lighter weights, as she takes you through many reps and changes the tempo each set to exhaust each muscle group (i.e. 3 count lift, 1 count lower for one set, then 1 count lift, 3 count lower second set etc.) There is little rest involved between sets.
In PS you get a little more rest and can use heavier weights since the sets are more straightforward and move a tad slower. Unless you are genetically inclined to "bulk up" I don't think PS will make you bulky. It's excellent strength training.
I love both of these workouts. PH you can do in one day, PS I usually spread over 2 days. I try to do them both in a week because I feel that they complement eachother extremely well. Ya gotta love Cathe!
Sorry if this is doesn't make too much sense, I could write a BOOK about these 2 videos but I'm at work and have tons to do.
HTH a little!
Liz