well I think studying english in a course is better, you'll have the chance to practice it with others.. I started learning spanish alone but i faced many dificulties then I joined some courses and I found it much better :)

What works best for you? That is a questions that only you can answer. I learn best learning one on one with a teacher and then applying that knowledge in a social setting. I can focus really well working with a teacher and then have fun speaking the language with my friends. This was the best for me, but it may not work for you. Focus learning is the best. What makes you focus? Use that, and then make your decision.

I think that learn english alone is very difficult but not impossible...If you want to study it alone, you should have a plan or strategy...

Studying with a teacher is better because the teacher is there to help you with any questions.

I had in mind to join some didactic book or some grammar book . But I think it should not to substitute a course .