Hi candipinki,
My wife and I recently spent 2 weeks at the Decameron. While there we visited (but did not stay at) Playa Blanca.
The Decameron is a very large complex - now nearly 1 km long after recent additions. Lots of nice pools and activities. We've stayed at nicer All Inclusives, but this one was nice for the 2 weeks we were there. While there we explored Panama a bit, using Decameron as our 'base'. The food wasn't quite as nice as other places but adequate and fairly varied. Someone told us that it had been purchased by an American company recently and that the attention to details had slipped a bit. Nevertheless, we enjoyed our stay there and felt that we were well treated.
Playa Blanca is newer and perhaps a bit more subdued. Fewer facilities and things happening but clean.
So it depends if you want a lively, bubbly place or a quiet vacation.
If you read the reviews on Trip advisor
http://www.tripadvisor.com/Hotels-g2...on-Hotels.html
or
http://www.debbiescaribbeanresortrev...panrevind.html
There you will find people who are happy and people who are frothing at the mouth because they're so mad about the bad things they experienced.
Try to average out these comments. Our experience in reading reviews is that every resort has good and bad points. If you are looking for bad things, you will inevitably find them and if complaining about them makes you happy, you will be smiling.
If you look for the good and positive things, you'll find a lot of them too.
Just go and enjoy and look for the good stuff!
(the above probably didn't help your decision making much did it?)