I need to use the Omega theme framework. I find there is two different versions: Omega 3 and Omega 4.
Which one should I use with Drupal 7?
There are Drupal 7 versions of both, so you can choose either. As of writing the 4.x branch is actively maintained, and 3.x not so much.
The 3.x branch will hold your hand a lot more, and make a lot of decisions about how you want to work. This may be or ideal, or may be frustrating. SASS support is clunky and a bit of a pain to work with, if I'm honest. Its main 'selling point' is that you can configure most of the options it supports through the UI.
The 4.x branch is much more of a developer-focused solution. Very little UI, but it incorporates a full front-end stack of tools for development/asset management. If you're a developer, this will likely be the one for you.
This is what the docs have to say:
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.
Beyond that, the only way you'll get a satisfactory answer is to install and try both.
Edit: to add a bit of balance to the above docs (thanks @FreeRadical for the info):
The 2nd point (claim?) is hotly disputed. It has been written by the Omega 4.x team who currently holds "ownership" over the project pages. This is a bitter dispute and there are negotiations currently taking place which probably will get the project page rewritten soon.
Sounds like fun :/
Both are created for Drupal 7, and both are responsive.
Due to an "disagreement" about policy and future direction within in the Omega development community, the project has - in effect - been split in two (Omega 3.x and Omega 4.x). These two branches are maintained by two completely separate groups of maintainers, and should be regarded as two different projects.
Omega 4.x is a base theme framework aimed at themers who want to gain full control over the theme through code, rather than a user interface.
Omega 3 is best suited to those who like to build their themes through the user interface.
Omega 3 is the original version and has about 55 000 installs for Drupal 7. It is stable and mature. It works well, but do not expect much new development.
Omega 4 is a recent development. It only has about 6 000 installs for Drupal 7. It is currently much more "dynamically" maintained than Omega 3.
Both are solid platforms. Which one to use depends on whether you like to do your tweaks and adjustments through a GUI (Omega 3), or interact with the code of templates and CSS or SASS (Omega 4).
On Omega 3.x vs. Omega 4.x - Comparing Apples and Oranges, you can read:
Whether you should use Omega 4.x or Omega 3.x really depends on how much you want to rely on the user interface for building layouts as this is no longer included in the 4.x branch. If you are okay with a possible lack of documentation and the fact that Omega 4.x does not come with a layout builder as 3.x did then you should definitely prefer the 4.x branch.
It is really a matter of personal taste/skills and of course the form of the project. I am using Omega 3.x, and it suits my purposes perfect. I don't have any really experience with Omega 4.x, and when i got between those two for a new project, after research and testing, I stick to 3.x, since it behaves much nicer (in my eyes) with very useful theming modules like Delta. As mentioned, it is a matter of requirements and personal view.
I propose you should take a look at the comparison done in the linked page.
At the time I am writing this, there are 3 branches for Drupal 7:
7.x-3, with a 7.x-3.1 version used from 47538 sites
7.x-4 with a 7.x-4.4 version used from 13144 sites
7.x-5 with a 7.x-5.0-alpha1 version used from 3134 sites
Despite the 7.x-4.4 version being suggested by the project maintainers, that is not the most used version, even though it was released on July 21, 2015 (3 years after the 7.x-3.1 version, released on February 19, 2012). The 7.x-5 branch will not have an official release (which means a version that is not alpha, beta, or a release candidate) before the 8.x-5 branch (whose development snapshot was last update on October 16, 2016).
So, after two years, the choose is still between branch 7.x-4 and 7.x-3, for which the most important difference is (IMO), the presence of a user interface to tweak the theme in the branch 7.x-3. If you are comfortable with editing the code of templates, CSS, and SASS files, then you can consider using branch 7.x-4, which was also updated more recently than the other branch.
If then you would be interested in updating to Drupal 8, you could consider the 7.x-5 branch, even though the 7.x-5.0-alpha1 version was released on October, 2014; it seems rather behind the 8.x-5.0-alpha7 version which was released on July 25, 2016 (almost two years later).
IMO, at this point, it is still too early for thinking about using the Drupal 8 version of the theme.