I haven't actually stayed at either but it would depend on your purpose for the trip. Are you coming just to be on south beach, do you even want to go to beach?

The Centric is good if the majority of your plan involves activities in tourist south beach area. Its not on the beach but you can walk across the hotel in front of it that is on the beach to access the beach.

The gables will be more low-key but you will still get a lot of good restaurants in the area and more central access to other things.