Buy your AP now and get a voucher that you activate in March.
This will save you any type of price increase between now and March. Also, once you have the voucher you can order your DDE cards ahead, a few days before you get there so you can pick it up first thing.
If you don't buy the AP until you're actually there, then it could take up to 24 hours after that before you can get the DDE card, thus you wouldn't have it to use your first day. (the DDE cards are processed off site, in batches, and then delivered to WDW. So depending on when the order first goes in, you could just miss a batch process, then have to wait until the next one which easily could be the next day)