If you're planning to be there that many days, I'd go for the AP, as the cost should be comparable, depending on how you're buying park tickets. If you're booking a package with room included, it may be tough to see exactly what the difference in cost would be to upgrade to an AP. I'd call and ask a CM to make sure.
The current AP 20% discount for dining is only at certain restaurants, and for the most part doesn't include character meals, but it can add up quickly, and the dining plan really isn't a "savings" in most cases. Like others have said, if you do your research, plan to eat at the most expensive meals (particularly character meals), etc. then you can come out ahead with the DP.
Go to the WDW site and search for the passholder benefits to see a list of the restaurants that accept the discount. Then just make sure there are enough on the list that you actually want to try. Remember you also get discounts on merchandise, so as long as you aren't paying a lot more for the AP, I'd go that route.