I think Anna is exactly right. I have been traveling to PDC almost yealy since 1990. It is sometimes surprising to return there and see the changes. For example, the beach at Porto Real had a real seaweed problem when I was there the year after it was built. This past April, the beach there was great. I have noticed that some areas of the AI resorts have a drop off at the waters edge as much as 3-4 feet. And at other times it is fine. The sand bar on the south beach area goes up and down.
Since I spend only 2-3 weeks per year in PDC, I can't offer an opinion as to why or how often it changes. I have read posts about this beach being better than that beach for various reasons. But peoples opinion of what is "best" differs and may not apply to you (or me).
What I can say for sure is that I have NEVER once been dissapointed with a visit to PDC. The changes it has undergone over the years is part of the local charm. I showed up this year and there was suddenly a fancy new mall by the dock, and the road was being paved over on 5th Ave BY HAND! Watching those craftsman work was amazing. You don't often see work like that done in this part of the US. I remember being outraged when a McDonald's and BK popped up. No matter how many times I visit, it always seems a bit different, but comfortingly the same. And that includes the beach.
As far as a hotel with an excellent stretch of beach??? I have stayed at the fanciest AI's as well as inexpensive non AI's. Every stretch of beach has been a great vacation. This year I stayed at the Playa Maya
www.playa-maya.com . This hotel is in the middle of the beach that you asked about, and it was certainly not washed away. Quite the contrary, the beach there was very nice and not as crowed as the AI's have been in the past.
I know this is not the specific direction you were looking for, but Anna was being honest (in addition to being correct.) Pick a hotel that is in your budget and go have a great time. No one area of the beach is so much nicer than another that it should be the deciding factor.
Enjoy your trip, and have a pina colada for me.