Absolutely in every case a music teacher! Especially if you want to become a professional. If you try self-learning you won't be able to become a professional, so now it depends on your goal - for the bedroom music instrumentalist, you might not need (however, I think every person wants to become better and better at their instrument, no matter of their goal). BUT, in order to stay motivated, you will have to find a teacher. A good pedagogue can't be beaten by nothing.
Also, if you find a teacher, it will be much easier for you to find gigs, as the teacher will be finding them for you. Another benefit from getting a teacher is you will likely find like-minded people who also share you passion for music and playing an instrument.
Not to say that finding a good pedagogue will not only teach you how to play, but also the ergonomics of the instrument. This is something extremely important, often overlooked by students, but in the end, this is what makes the good instrumentalist good, and the bad instrumentalist bad.