For teaching the game, I think the Beginner Sets for Star Wars are pretty dang good. They are fairly short scenarios, and gradually introduce various concepts: this is how a skill check works, this is a simplified fight, this is how opposed checks work, here's a more complicated fight where we'll use the actual initiative rules, this is how you advance a character, and so on. In addition, they come with web enhancements that let you easily build on the stuff in the original adventure for a bunch more adventuring – "What's next?" is a vital question to answer in any beginner adventure.
As to whether a standalone beginner set or just a scenario is better, that's more a matter of taste. A beginner set usually offers a low-cost introduction, particularly for the bigger/more expensive games. This is good for people who want to try it out. On the other hand, it means it likely contains some material duplicated from the game's core rules, which some consider "waste".
I'm not in favor of including it in the core rules, at least not as part of a single volume (it'd work great in a boxed set). I would not want to carry the intro adventure around wherever I take the game, and it's a nuisance to flip back and forth between adventure and rules – particularly for a newbie who doesn't have the rules down yet.