Currently there are two versions of Drupal available for download: Drupal 6 (D6) and Drupal 7 (D7). There are several aspects to consider when choosing between one version, or the other.
Drupal 7 is newer and will be supported by the Drupal community for a longer period of time than Drupal 6. Drupal 8 is scheduled to be released at the end of 2013, at which point Drupal 6 will no longer be supported, nor receive security updates.
Drupal 7 contains more features and improvements than Drupal 6.
- Usability: the user interface in D7 is now more intuitive thanks to a simplified menu structure, an administrative overlay and a customisable dashboard.
- Performance and scalability: D7 is considerably faster than D6 when many modules are enabled.
- Flexibility: D7 integrates into core some functionalities previously provided by other modules.
Some contributed modules do not have a Drupal 7 release, either because they are not maintained or because their functionality is provided by other D7 modules. This may be a factor to consider, especially when planning to upgrade existing D6 sites to D7. However, according to usage statistics for Drupal core, D7 is already being used in more sites than D6, which is gradually fading. Also some new modules don't have a D6 release.
Generally, if you're building a new site D7 would probably be the recommended choice. However, if you're planning to migrate an existing D6 site to D7, you should be aware the migration process may not be straight forward, depending on the level of customisation of the site.