Export and import content from one drupal site to another? I want to export content with comments and user data with content relation from one Drupal site to same version of another site. In another site there is no content and users data it has only some new module installed and it's configuration. 

For this which is best way migrate/ node_export/ Backup and Migrate
If you're exporting into an empty site, the Backup and Migrate module is the one you want.  It will let you clone all contents on a Drupal site, including comments.  (Or only certain tables such as thos associated with nodes and comments, if that is what you want).

The Node Export module will only let you import/export nodes.  Comments, for instance, is not nodes.

The Migrate module is a more general framework for migration (including migration data not originating from Drupal into a Drupal site).  It gives you a lot more control over the process, but I think it is overkill if all you want to do is to export and then inport into an empty site with the same version of Drupal.  Incidently, if you still want to use Migrate, check out Drupal-to-Drupal data migration.  It is based upon Migrate, but designed to specifically migrate (and refactor) data from one Drupal site into another.