Should I use 3.x or 4.x?
Whether you should use Omega 4.x or Omega 3.x depends on several considerations:
- Do you rely on the user interface for building layouts, or are you comfortable defining layouts in code in a tpl.php file? Omega 4.x does not include the elaborate layout definition UI that was a hallmark of Omega 3.x
- Do you want to use a theme that is under active development, or are you comfortable with a theme that is minimally maintained? Omega 3.x is minimally maintained. In all likelihood, unless a new maintainer steps up to the task, Omega 3.x will not be upgraded to Drupal 8.
- Are you comfortable with or willing to learn SASS, or do you want to stick with straight CSS? Omega 4.x does NOT require use of Sass but certainly is much easier to implement if you adopt Sass.
This issue is a thorough discussion of the changes in 4.x.