The next big version of Drupal is launching on November 19th 2015. This also means that the end of line for Drupal 6 is set to February 24th 2015. After that date it will not receive any updates. Unlike some Linux distributions, like Redhat the system does not have a third party backer to backport fixes from Drupal 7 or 8 to version 6.
Similar to Windows XP, the use of Drupal 6 will continue long after the support has ended. But you really should upgrade, but to which version? Drupal 8 is a large change from the backend compared to Drupal 7, so a lot of the code that was feasible to transition from Drupal 7 requires a complete rewrite on Drupal 8.
Should I upgrade to Drupal 7 or 8?
Drupal 8 comes with a handy migration tool that allows migrating data from versions six or seven to eight. While this will handle data, the nature of Drupal as a platform is to rely heavily on third party extensions (called modules in the lingo). There are a limited number of these available at launch time for 8, but the situation is expected to improve throughout 2016.
If you are running a lightweight, content driven site on Drupal 6 with few modules in use, you can likely transition that to Drupal 8 with reasonable work. If you're running a heavily customised one, with lots of modules and complex integrations you're likely better to upgrade to Drupal 7, which has plenty of life in it through community support. Drupal 8 is mostly a developer focused release, while more end user features are in point releases 8.1, 8.2 and so forth.
A complete rewrite of a site built in 2010 to technology in use in 2015 is a big task. This is something that needs to be done and does offer some benefits, but it should not be taken lightly. There are plenty of things to consider when doing content migrations between Drupal versions, so put this on your list of things to be done at a longer term.
In a nutshell:
- Upgrading a complex Drupal 6 site is probably best done through an upgrade to Drupal 7
- A simple content driven site with limited module use (multilingual, multisite is ok) is a good candidate for Drupal 8 upgrade