Sample ticket prices (adult theme park admittance only - no water parks or DisneyQuest) ...
Undercover Tourist @ MouseSavers.com (Price includes tax + free shipping) -
6 day MYW(No Exp) - $307.95
7 day MYW(No Exp) - $355.95
8 day MYW(No Exp) - $394.95
10 day MYW(No Exp) - $452.00
6 day PH(No Exp) - $371.95
7 day PH(No Exp) - $402.00
8 day PH(No Exp) - $446.95
10 day PH(No Exp) - $501.00
1st year AP - $520.79 less $100 DVC discount = $420.79
(unlimited admission to 4 major parks)
Assuming my numbers are correct (prices subject to change) then the choice would depend on whether you wanted the Park Hopper option. If you do NOT, then the AP is not beneficial unless you will spend 9 days or more in a year at the parks. If you want the Park Hopper option, then the AP is better if you will spend 8 days or more in a year at the parks. Prices are different if you wish to include the water parks and DisneyQuest.
In my case, I will be at the parks for 3 days in May, 2010, 4 days in Nov, 2010 and 3 days in Nov, 2011 so I am getting the 10 day Park Hopper with the No Expiration option.
Of course, ticket prices will change in a year or so and we'll have to recalculate the choices all over!