I think it's going to depend on how many days you are planning to spend on disney and if you want the water parks too.
Adding the water parks option for the 10 day ticket w/no expiration is only $45 and you can go five times to the water parks. With the
DVC discount, you will pay $30 each time you go to a water park.
There are other things to consider, such as if you want to park-hop, which is an additional $35 in your ticket w/no expiration.
In my particular case this year what we did is we bought one seasonal pass (I live in FL) and with that we get the DDE. Then, since we are going in thanksgiving with only three days for parks and in October 06 for 5 days of parks, we bought 10 days MYW w/no expiration, park hopper and magic plus for the rest of the family. We will use 8 (or maybe less if we go to other non-disney parks) of those days this year, and still will have 2 more days + any water park pass left for 07 forward. We would have saved $105 per adult on water parks tickets and will not be pressured to go to disneys parks only because we have the AP instead of enyoing some other Orlando attractions.
But again, the final answer will depend in how you usually spend your time during vacations. If you are one of those that have to hit a disney park every single day of your visit, including arrival and check out date even if it is for two hours only, then you are better of with the AP so that you don't feel you are "wasting" your MYW days.
But, if you are like us and like to enjoy the best that Orlando has to offer outside of disney you should really give it a thought to the MYW w/no expiration which could be there for you for two or maybe more years. It's like DVC, you will make a larger investment right now, but it will save you money in the long run and you will not feel pressured to make decisions based on the expiration date of the AP.