rutgers1 - There really aren't any hard and fast rules for getting an AP vs a MYW ticket. As BirdsOfPreyDave posted. it comes down to how you plan to visit the parks and how many trips you plan to take within a year. For example:
DVC adult AP is $370.62
DVC adult PAP is $483.51
A 10 day MYW PH ticket that expires without plus options is 287.55
A 10 day MYW PH ticket that does NOT expire & no plus options is $479.25
A 10 day MYW PH ticket that expires but has plus options is $340.80
A 10 day MYW PH ticket that does NOT expire & has plus options is $532.50
All listed prices include tax (and you may have been able to get a discount if you bought in advance from an authorized Disney ticket broker).
Anyway, if your 10 days are spread out over more than one trip within a year, the PAP would be a better value. The MYW ticket that expires would be better if you used all 10 days on the same trip.
If you could get by without visiting a major park on the day or days you spend in a water park, you could get a less expensive MYW ticket.
Depending on how many of the plus options you use, it may be a better deal to get the AP and pay extra for the water park or PI visits. Again, it just depends on how you plan to tour.
With the advent of MYW ticketing, you can't really do the math without a lot of specifics about the guest's park touring plans.