Extreme IV or Extreme III

I am feeling very happy when I saw the compact flash words in one of my assignment actually. I want to buy compact flash cards for my camera since last few weeks. However, I am having problem to select which type of compact flash to buy. I am interested in the Extreme IV compact card, but the price quite expensive. I am not sure is it worth to buy. Alternatively, should I get the cheaper price for Scandisk Extreme III compact flash card? Hard to choice! I still have no answer until today. *sign*