Power Hour's lower body workout is tougher given it has a ton of reps that seem to never end. I think Muscle Max's upper body work is tougher than Power Hour's. Also, the ab work in Muscle Max is tough!!!
You know, it's like apples & oranges to me. I love them both b/c they have different exercises I really enjoy. If you want to just add one to your collection, I would start w/ PH b/c it also has MIS & bodymax which are great!!
ETA I don't know if you're actually buying something, but I would start w/ PH
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