Well for the question your asking I would say a teacher is better always. If you were asking if a teacher is NEEDED, then no, not necessarily. You can learn perfectly fine on your own, but with a teacher you can learn your mistakes faster, which is what I feel the benefit is. He will be there to correct your mistakes right away (hopefully), so you don't develop bad habits. Plus he can show you things you may not learn till a lot later if you were to teach yourself