Stability of JSON textual representation

I've noticed that when representing lists as JSON, Postgres 9.4 sometimes outputs spaces after commas, and other times does not.

# SELECT array_to_json(array[1, 2, 3]), json_build_array(1, 2, 3);

 array_to_json | json_build_array 

---------------+------------------

 [1,2,3]       | [1, 2, 3]

This matters to me because I'm intending the hash the JSON text and use the hash for change detection.