If you are only going once a year, get the 10 day. With a 10 day you get 10 days in the parks plus 6 water park and fun days + no expiration. That's 16 days of fun. Our family does not go to the parks every day so a 10 day ticket will last us 3-4 trips. The cost is $421.75 or $408.97 from Disney if you order ahead. That 's about $25.55 a day at the parks and it cost you nothing to pool hop, go off site (we never do), site see around the world. An annual pass the first time is $441.98. If you go twice in one year for 8 days your cost per day is $27.63. Not much of a difference except an AP is only good for your two trips where the day ticket is good for 3-4 trips; for us atleast. (All prices are 2006 prices from mousesavers.com)
I guess it all depends on how you plan to spend your days. If you go to the parks everyday, then the AP is better. If you only spend a couple and like the water parks 10 is better. If you are also looking to save money down the road, the 10 day is a better deal.
Again, we only spend 2 days at the parks (sometimes 3) and go to a water park or two each visit. We go to WDW to relax and enjoy freedom not to go comando every day. We don't see the benefit of being able to walk into EPCOT to have dinner after spending the day at the pool. We maximize our ticket money by doing it this way.