It is not a very difficult question. If you want to work on your English (and any other language) at your own pace individual classes are for you. Your teacher should guide you through the learning process. The course/classes should be tailor-made to your objectives, expectations, and your current level. If you are willing to do your homework, you can make big progress in a reasonably short period of time.
Group classes impose certain limits. Because they are usually provided by the language academies, there are a few fixed objectives a teacher has to/should reach. Students pay for their course books (less frequently workbooks these days) in the form of a physical copy or an online one (via an online platform for example) and so teachers are obliged to work with such resources. If I were asked to buy a coursebook that would not be used much in class, then I would feel cheated to a certain extent. In group classes, you usually have a group of students who, at least in theory, should be nearly on the same level and if most of them attend the classes regularly there is a degree of dynamism and variety that is an added value for everybody. Because every group course, be it a conversational, business or technical one, entails doing a certain amount of material in a designated period of time in a very structural way, the pace can’t be adjusted to an individual student. On the other hand, it is easy to play games and do group activities when group classes take place.
Group classes can be both fun and funny for students as much as the teacher. Individual classes can be the same but because the focus is always on that one particular student it can be very draining at times for both the student and the teacher.