Note that you do not need to have an AP in your possession in order to book a room with an AP discount rate. You just need to have the AP in your hand when you check-in at the resort.
So, you could defer your ticket purchase decision until just before your trip. If an AP room discount is going to impact your decision, just wait until very late in the game to purchase your tickets. If an AP room rate is offered, book the AP rate and then pick up your pass before you arrive at the hotel (either at Downtown Disney, the airport, or a theme park).
If you decide not to go with the AP, you can still get the "Advance Purchase" ticket discounts just a couple of days before your trip by ordering on-line via Disneyworld.com and selecting the "Will Call" pickup option.
I wholeheartedly agree with SnackyStacky--don't count on any discounts until they materialize. There may not be any discounts offered for your travel dates. If offered, the discount may be less than you were anticipating, or you may not be able to get a discounted room at your choice of hotel / accommodation--there are only a fixed number of rooms reserved for AP discounts when offered.
One final thought on the AP--an annual pass will be good for 365 days (actually 366) from the date of issue. So, if there is a high probability of your returning to WDW in the next 12 months, the single AP could cover your admission for both trips. We live in Ohio and will get at least 3 trips out of our current APs--May '04, Dec '04 and May '05.
Good luck.