Zen vs Omega

I came across your question because I was looking for an easy way to convert an existing Zen sub-theme to Omega (which doesn't look like it exists).

In my opinion Omega trumps Zen because it integrates with Context + Delta. if you have a large site with a lot specific layouts, and maybe page specific CSS or Javascript you want to use, you can't get any better than Omega.

You don't need to do any custom template theming with this, and it's much easier to test layouts, and roll back layouts you don't like through Delta. Where as Zen requires you edit all the core template files if you want to make a custom theme. With Omega, if you want to add custom CSS libraries you just edit the template.info file, then with Delta you can decide if those files should be included on every page, or only a specific page without having to make custom functions or additional templates.

That's a huge gain in the flexibility of your layouts. Omega trumps Zen because of this fact alone. That's my opinion at least.