If you are going to be there 8 nights or more, the AP makes more sense than any other ticket medium.
There are some discounts associated with the AP...another poster made reference to a link, so I won't repeat that.
The price difference between a regular AP(4 parks only) and the PAP(4 parks + PI and water parks) is about $120, I think. Look at the admission price to PI and water parks...if you don't think you're going to do $120 worth of admission to any of those "extra" places, it might behoove you to think about a regular AP and then purchase day admits for PI/water parks.
DH and I are not big PI people...we usually go once on a trip, maybe twice at the most, so we just purchase a one time admission when we get to PI. We generally don't go to the water parks, because we are there usually in the cooler months when either BB or TL is closed for rehab, and the weather is not atrociously hot, so the hotel pool works fine for us.