The process of learning guitar IMO can be best described by the phrase "Leave no stone unturned". So to answer the title of this post I would say you should do both... ALL. But having said that let me point out some merits of having a teacher.
Here are the conditions where I feel one NEEDS a face-to-face teacher... or at very least the presence of one can greatly affect the time it takes.
Anything that comes with a LOT of questions/confusion (i.e. theory).
Sure you can learn theory online - but it can take YEARS instead of months. (See below for an expansion on this)
- When you have a technical issue(s) and need some technique adjustment.
- When you are a person who needs to have someone you are accountable to. Some don't need this, others very much.
- When you are a visual learner. NOTHING beats sitting in the room and watching someone play.
When you feel stuck in a rut and really need some new input to bust out of it.
I would recommend you go to some local shows and see some local players. When you see one that moves you ask him/her for a lesson. Stay with them until you find out their "stuff" then move on ... or stay whatever.
I will say this, one of my biggest regrets as a player was that I was never able to find my mentor... That person... Sure I took lessons, but never for very long. There is a HUGE value in the inspiration that can come from sitting with a great player. For example, during any lesson I get with Oz Noy, it is incredibly humbling and inspirational to be 'put in my place' so to speak. Also I can chat with him about little theories or observations I have. Nothing is better than having a great player CONFIRM something you always suspected.
Finding the RIGHT teacher is HARD... but imo it is worth every minute of the search. If I could do it all over again I would spend much more efforts in finding someone who simultaneously inspired, taught and pushed me... They are out there.
The Case for Theory
I personally do not see theory and playing as the same process at all. One (music theory) is cold calculated observation and groupings. The other (playing the instrument) is more of an approach/technical/magical thing - lots of grey area here. To learn to PLAY, a great teacher is a HUGE asset. To learn theory, almost any garden variety teacher of any instrument (who is qualified) can walk you through the Conservatory text and workbooks.
I stand by that for me THE BEST THING I ever did HANDS DOWN, was go to a teacher who forced me to go through my Level One and Two Rudiments (the teacher MADE ME sit the exams and the whole drill). I didn't even bring an instrument to the lessons until I was done. The foundation that this laid for me, is one that I draw on to this day. It has allowed me to self-teach and generally explore any area of music that interests me. But from an informed place. At the time I sat the exams, I didn't even understand how to apply a lot of the stuff I was learning. But in the years since, the dots have slowly connected. I can't put a value on those lessons and the wonderful foresight provided me by a wonderful teacher. Thanks Ray! You're the man brother.
The best thing for any musician is to find a more advanced player to help them along. Or several people. Sure it can be costly, Yes it can be inconvenient making the lesson time. Finding the right person can cause frustration and time in the search (not to mention money). But you have to ask how important playing well is to you. If you are just a hobby player and really don't care, then fine, learn online or from books. But if you are really serious about it then adding a great teacher to all that can be a difference maker. It really is up to you.
The Role of the Student
I think students would be surprised to know how important they are to the process. Critical. When a student comes through the door, prepared and excited it makes the teacher respond in kind. I was an AWESOME teacher for people like this... but I think most teachers would be too. I was simultaneously an ashamedly terrible teacher for other students.
If you go to your lesson with excitement and an idea of what you want to learn, then you stand a far better chance of getting your money's worth. As opposed to slinking in the door and expecting the teacher to knock your socks off while offering nothing.
The process of learning guitar isn't necessarily linear ... a lot of newbs expect the teacher to progress step by step... Sometimes you can, but often times there is one of a hundred different directions you can go. I used to start every lesson the same way - review last lessons materials, answer any questions, make any adjustments to technique then the dreaded "So what would you like to work on this week?" To which MANY had ZERO answer. I mean honestly, you have ALL WEEK to think about what you would like to do... How can people have ZERO idea? I'll tell you why, it's because they don't care, don't practice, no spark, waiting for someone to do it for them. The kicker is these cats walk around and slag that they got nothing from the lessons. I used to fire students all the time. "Come back when you are ready to learn."
If you expect ANY teacher to turn you into Jimi Hendrix you can forget it. Hendrix turned Hendrix into Hendrix. Just as you will turn you into you. It's up to YOU.
My advice - if you are taking lessons, be ready, be excited, be enthusiastic... Mostly be prepared to do the work. If you can be all these things I suspect you will have a hard time finding a bad teacher.