Definitely Grand Bliss because deck is larger and you get to use the Beach Club, Grand Mayan, and Mayan Palace pools. I say this because the Grand Luxxe pool there is not really beachfront and we love sitting around a pool and being able to also see the ocean. We have stayed at MP and GM there, and loved the Palapa beds with the pool on one side and the ocean on the other. Our family ranks the Mayan Palace pool at Riviera Maya as one of their favorite pools.
The only negative for me is that the Grand Bliss has 2 under-counter refrigerators instead of a real refrigerator, with some freezer space, a sofa & loveseat instead of 2 sofas, and only 2 drawers in each of the bedrooms - I do unpack our clothes when we arrive and lots of things are not hang up, so one drawer each is not ideal.
Regardless, we do have 2 weeks at GBMR reserved for late April/early May 2018 and are looking forward to going there again since it has been quite a while since we have been there. We used SFX exchanges using PBEB weeks and I chose GB because of the Beach Club pool access.