I think you need to plan and decide which parks you want to visit before you go and compare the daily entry charges with a 7 or 14 day pass.
We got good value from the Universal passes. They are a lot cheaper than the Disney ones, and my family is older than Sherrardks family and the new Harry Potter area and new Simpsons area were 'musts' for us. We also got good value from the Disney passes. I got mine tickets from Travel Republic before we went. Which area are you staying in?