Never choose music lesson / tutor because it's cheap. It's a complete waste of time to learn with bad tutor. Qualification and certificate is very important. At least it can show that the tutor has passed formal professional training. Unlike other classical instrument, very little guitar tutor has taken exam like Royal Exam and Trinity Rock school (http://www.rockschool.co.uk).
It's also a great idea to take private lesson as time is more flexible and also good tutor won't teach in 琴行 as 50% of their tutor fee will be taken.
You can contact 導師街 (www.yourmusictutor.com) and try to ask them to recommend a few professional tutors for you. These tutors are from music schools and usually teach private lessons and in schools which you cannot find in 琴行.