It depends. Will you visit WDW between Jan.-May 2015? If not, it really won't be an advantage to purchase an AP on Memorial Day weekend instead of renewing now. The AP would go unused during that extra time period. It would be smarter to renew now and save yourself the cash.
Look at it this way - you can renew now for $420 ($526 for a PAP) or you can spend $495 ($621 PAP) in May. If you renew, then your pass will expire Dec 23, 2014. If you don't, then you won't be able to visit between now and Memorial Day of this year, but you will be able to do so during the same timeframe in 2015. It really will depend on what you anticipate when your visits will occur over the next 18 months.