.

The goal of any study plan is to prioritize, focusing on those areas in which you have your greatest weaknesses. For example, if you recognize that you have a particular weakness in Data Sufficiency, you should focus on this topic by working through the OG Main and OG Math guide particularly in relation to data sufficiency. In this situation, you might want to work through all problems in the OG Main in relation to DS, check them and analyze what you are consistently getting wrong, revisit this strategies and methods, then work through all of the problems in the OG Math to see if you have improved. In any case, you should always spend the majority of your time on your weaknesses.

In relation to the difficulty of a given book, the difficulty level is probably about the same between all of the OG books. Difficulty, in this case, will mostly depend upon your proficiency in a given topic. Just remember that practice makes perfect.