Should you upgrade Drupal 6 to Drupal 7 now and wait for a few years, and then upgrade to Drupal 8, or should you wait and upgrade Drupal 6 to Drupal 8 directly? Here are some "if's" that will make you want to wait. You may want to wait, if...
- Your Drupal 6 site theme is outdated and needs to be redesigned and reworked heavily. Why? Because in this case, your site will require a serious investment to update, so you may want to wait and upgrade to Drupal 8.
- Your Drupal 6 site has lots of php and styling in content. Why? Because in this case, your site will need lots of tedious work cleaning your content. So you may want to wait for a year or two, and join that cleanup with migration to Drupal 8.
- Your Drupal 6 site has lots of custom modules. Why? Because custom modules need to be rewritten, adjusted. It would not make sense to invest into Drupal 7, if a newer version will be available in a year or so.
- Your Drupal 6 site is not important, you rarely update it, it just "exists".Why? Because you are probably not using your site's potential anyway, so why upgrade? (Better develop a web strategy!)
- You are short on money and won't be able to handle two upgrades within the time span of 3-4 years.Why? Because if of two upgrades, you will only be able to choose one, then choose the Drupal 8 upgrade.