Do you plan to be at the 4 main parks every day? What happens if you cancel a trip or shorten one of them for one reason or another (you never know...). There are other perks, especially the free parking one (which doen't apply if you are staying on grounds, there are some restaurant discounts, and there is a discount to stores at Downtown Disney (only if you make a $25 min purchase each time to get the discount). There are also other perks, minor basically, as I recall (we usually get the Florida Seasonal Passes, but sometimes will get the AP's if we're going at peak times). You need to decide if it's worth the gamble. You are betting (by buying the AP's), that you are going to spend at least 8 days in the park over one year, and if you are pretty sure you will spend at least 8 days in the parks, it IS a good deal. If it will be less then 8 days that you intend to stay in the parks then go with the park hoppers.
I am not sure, but I think that you can upgrade your 5 day park hopper to an annual pass, does anyone know this for sure? If you can, I'd consider upgrading once I was more certain of my plans.