It is better to have professional English teachers teaching the language than merely having native speakers. Although native speakers are competent at the language, professional teachers know how to teach better. They have had to go to school in order to learn teaching methodologies and classroom and student management techniques.
As a person who has taught English in two countries now, I can tell you that the professional label doesn't necessarily mean the teacher is better. The best way to teach is to just show them basic words and how to have conversations. Memorizing specific sayings and whatnot is not the way to go.
No, professional English teachers are not better than native speakers who teach, because the easiest way to learn a language is to talk with someone who speaks it. Someone who speaks a language natively will know more of the slang and catch phrases that are actually used in daily speech.
I personally think that professional English teachers and native speakers who teach are two different teachers that teaches about English. I personally think that professional English teachers are as great at doing their job which is teaching as well as the native speakers who teach. I personally learn from both English teachers as well as native speakers who teach.
I do not believe professional English teachers are better than native speakers who teach. As a native speaker of English I have learned far more from other native speakers about the proper ways to speak and write this language. I learned after having a hard time in elementary, middle, and high school with professional teachers.