If you know for sure that you'll be returning (and not booking a package), then getting the 10-day no-expiry passes is a good idea as long as you have the money to do it now. If you're going to put the purchase on a credit card and carry a balance for several months, the interest that you pay will negate any savings you may have realized.
My suggestions for places to buy tickets:
Compare their prices, including tax and shipping charges. And if you sign up for the
MouseSavers.com Newsletter (which comes out on the 15th of every month), you'll find links in it for even lower ticket prices at one or more of the above ticketbrokers.
All of them are legitimate and authorized sellers of Disney World passes. My experience has been that
AAA does not offer every ticket type and they are not cheaper than the above-mentioned sources. I've used Ticketmania, UndercoverTourist and DiscountOrlandoVacations myself and have no complaints. The others come highly recommended by other DISers.