What is the difference between BRAM and distributed RAM
Block Ram is a dedicated Ram that does not consume any additional LUT in your design whereas distributed Ram is built up with LUT. 
In terms of speed the distributed RAM is faster than Block Rams. 
Generally speaking, if not much Ram is needed you can consider to implement it as a distributed Ram. Some synthesizers may even use distributed Ram if you specifically chose Block Ram but just use a very small amount of a single block.
As soon as you have large Ram blocks it doesn't make much sense to use distributed Ram.