Yes, 12mm is a very big difference from 20mm on the a6000.

The Sony 20mm f/2.8 mainly gives you a smaller, lighter package and a modest speed advantage over the kit lens at 20mm. But that focal length is already covered by your kit zoom, so it doesn’t really add a new look—just a more compact way to shoot it. For daylight travel photos, where you’ll often be at f/5.6–f/11 anyway, that advantage is fairly small.

The Rokinon 12mm f/2 gives you a much wider view that your kit lens cannot match. That makes it more useful for dramatic scenery, tight spaces, and travel shots where you want to include a lot of environment. It’s also fast at f/2. Manual focus is less of a problem on an ultra-wide lens, especially for distant subjects.

So the tradeoff is:

• 20mm: compact, convenient, but overlaps your current lens range
• 12mm: much wider, more creative, adds capabilities you don’t already have

For your stated goal of vacation photos with people and scenery, the 12mm is the more distinct and useful addition if you want something your current lenses can’t already do.