Both of these expressions are used interchangeably and would be understood to have the same meaning. "Milling (or wandering, walking) around/about" both suggest casually moving within an area. In general use, neither suggest a particular route or method.
However, "around" can be used to suggest a broadly circular route following the outer perimeter of something. For example "I walked around the block" would be taken to mean someone walked a single route that took them around a block of buildings and they ended where they began. But, words like "milling" and "wandering" imply some degree of aimlessness, so used with those, this would have a different meaning. "Wandering around the block" sounds like someone just moving randomly within the area.