If you will have large gap between the date the AP expires and your next vacation at WDW, it might be better to wait and just purchase a new AP. For example, our annual passes expire February 28, 2011, but out next trip is not until June, 2011. We do not plan to renew our AP because we want the AP year to end in June 2012, not February 2012, so we can get three visits to WDW on the same AP.
An regular AP for adult costs $531.44 less the $100
DVC discount = $431.44
(a child is $479.25 - $100 = 379.25)
A renewal costs $488.84 for an adult and $441.98 for a child. I believe you still get a DVC discount on top of the discount, otherwise, no DVC member would bother to purchase a renewal. Can someone confirm that DVC members get a discount on a renewal too?