I think the key distinction is what each resource is actually designed to do.
Official GMAT material is useful because it gives you exposure to authentic GMAT question construction and provides official practice exams for assessing your readiness.
But I would not view official material as a complete preparation system, especially for someone starting from scratch.
The Official Guide does not provide a comprehensive learning path, deep instruction, or enough targeted practice to thoroughly develop many individual skills. In fact, the number of official questions available for some specific topics and question types is quite limited. If you have a weakness in a particular area, doing a relatively small number of official questions is unlikely to be enough to build mastery.
That is where a comprehensive course can be much more valuable.
A strong course should teach the underlying concepts, strategies, and processes systematically and then give you enough focused practice to turn those concepts into reliable skills. If you struggle with rates, overlapping sets, Critical Reasoning assumptions, Two-Part Analysis, or another specific area, you may need dozens of carefully selected questions on that skill rather than the handful you might encounter in official material.
Another common mistake is using official questions as the primary vehicle for learning. You miss a question, read the explanation, understand why you got it wrong, and move on. But understanding the solution to one problem is not the same as developing a skill you can repeatedly execute under pressure.
So, if I were starting from scratch, I would think about the resources this way:
Use a comprehensive course to build your knowledge, skills, strategies, and consistency.
Use official GMAT material later to gain additional exposure to official questions and, most importantly, use the official practice exams to assess whether your preparation is translating into the score you need.
For most students with meaningful weaknesses, I would not rely on official material alone. There simply is not enough instruction or sufficiently deep topic-by-topic practice for that to be the most effective way to build a strong GMAT foundation.
Good luck!
- Scott, TTP