Another element to consider is the Tables in Wonderland. If you have an AP, it costs $75, and you can buy it in DTD where you cna also buy your AP (or exchange your voucher). Though the line in DTD can be slow. The TiW gives you 20% off at most TS locations (including signature locations and alcoholic beverages - for parties up to ten people!) Just an AP gives you some discounts at certain hours and locations, including merchandise at WoD.
As other posters have said, general public disneounts are aften just as good, but they also have their own restrictions, and those restrictions may not work for you. Like
free dining kind of requires you be on WDW property to eat. If you are going to Universal - a room discount may be more useful.
I think the other thign to kind of consider is that the savings has to cover the added cost of an AP. That is easier to do if you are staying at a higher end resort. If you normally stay at a value, then odds are good that the gen public discount will be just as good or better.
You can't relaly do the math until you know the discounts. Also, the AP discounts tend to come out rather late - sometimes after you've already passed the 45 day window on a discounted package. (packages must be paid in full 45days out- and changes after that involve paying a penalty.)
AP discoutns tend ot work for last minute trips. The other thign to consider is if you will be having multple trips in one year. If you are rather certain you'll be back at least once within a year, then an AP becomes more tenable. (You can make first trip say Dec 8- 15 this year, and next year's trip Dec 1-7.)
There is a discountt usually on renewing an AP.