IELTS General and Academics are the two sides of the same coin, each with a different purpose and marking criteria. The General module is a more general test which assesses your overall English proficiency. The Academic module, on the other hand, focuses more specifically on academic English and assesses your proficiency in reading, writing, listening and grammar–based tasks.
Some people believe that IELTS Academics is more difficult, but when you look at the mark scheme in relation to their complexities, they are roughly the same in terms of difficulty level.
The following is a breakdown of the different parts of IELTS general vs. academic.
Speaking and Listening:
For speaking and listening, both tests have the same duration, structure, and scoring criteria. There are no differences at all.
The academic portion of the reading test is much easier than its counterpart. However, the difference lies in their grading criteria. The number of correct answers you need for the same band in a general exam is much higher than in the IELTS academic. For these reasons, you can assume that the difficulty level is more or less the same. Aside from that, the structure and the question types are the same, with the exception that the passages in IELTS Academics are more difficult.
This is the only section with significantly different questions differ. Part 2 is the same in both exams. Part 1, however, differs significantly. Part 1 of General IELTS requires you to write a formal/informal letter, whereas the General IELTS requires you to explain a diagram. Some consider the explanation of the diagram to be difficult, but I believe that they each have their own subtleties and therefore are on the same level of complexity.
In summary, both exams are administered by the same parent organisation, IELTS, and therefore share many similarities. I’d argue that 90% of the difficulty level and preparation material can be considered the same.