Easily upgrade from D6 or D7 via the D8 core migration modules, before deciding whether manual copying might be better.
Prepare D6 or D7 for upgrade to Drupal 8: Pre-migration details
Upgrade to Drupal 8 from D6, or D7 via UI: Using the built-in user-interface for migration
Drush Upgrade of D6 or D7 to Drupal 8
NOTE: A more complete example module can be found at https://github.com/heddn/d8_custom_migrate.
The following is a list of various items that may require special attention when migrating from Drupal 6 and 7 to Drupal 8.
This is a stub page referenced from update.php in Drupal 8 core. Instead, please see the page "Upgrading from Drupal 6 or 7 to Drupal 8"