Hi Amy,
I'm obviously not Cathe or an expert on the matter, but I think the general concensus for building muscle is to work each body part (using the heaviest weights you can handle with good form) 2 x per week and no more than 3 x per week.
If you are looking for maximum muscle building, then I think S&H is what you should be doing. I believe most people do each S&H tape 1 x per week as it really is an intense series, and tends to catch up with you about 3-4 days after doing it. The thing to remember is that the rest time between heavy lifting is just as important as the doing of it. This rest time is when the muscles are repairing and becoming stronger, so doing too much can inhibit your results.
You don't mention how long you have been working out or which particular strength series you have been using, but if you are doing 31/2 hours of S&H each week, I would suggest that this would be adequate. MIS and Power Hour are more "endurance" based (ie shorter rests/lighter weights/longer reps) so whilst you will get some good results with these, for maximum muscle fibre recruitment, I think you are looking at the S&H series.
I know that some people do S&H 2 x per week, so maybe they will read this thread and give you their experiences of this.
As I say, not an expert, but just my opinion. Hope this helps.
Kaz.
