Add an HNG index when you need to do range searches.
An HNG index requires approximately three times less disk space than an HG index requires. On that basis alone, if you do not need to do group operations, use an HNG index instead of a HG index.
Conversely, if you know you are going to do queries that a HG index handles more efficiently, or if the column is part of a join and/or you want to enforce uniqueness, use a HG index.
Using the HNG index in place of a HG index may seriously degrade performance of complex ad-hoc queries joining four or more tables. If query performance is important for such queries in your application, choose both HG and HNG.