Hello Lynn:
I have MIS, Pure Strength and Slow and Heavy and love them all.
If you only wanted to get either PS or S&H I strongly recommend the Pure Strength. It is the cornerstone of my exercise foundation and can yield awesome results. A good rotation would be to incorporate all three Pure Strength tapes and MIS in one week. That way you can target all muscle groups twice in a 7 day period, but in different ways.
Slow and Heavy is an excellent add-on. I believe it was meant to be a system used in smaller rotations, such as 3 weeks, to challenge your muscles in entirely new exercises. That would help you to gain even more strength and that in turn leads to more muscle development.
Personally, I use Pure Strength/MIS (as well as CTX upper body) for ten to twelve weeks, then do Slow and Heavy for three weeks before going back to Pure Strength. Slow and Heavy is so intense that I don't think I could use it for more than 3 or 4 weeks at a time.
So, if you can get both Pure Strength and Slow and Heavy I think you should do that. If you only want one, I vote for Pure Strength.
Marlene