Which is the better strategy in essay type exams?

While writing an essay, there are basic tips and structures one can follow to create an effective and eloquently written essay that will score you the most points in an exam. 
Make sure to read the prompt thoroughly and correctly. A small mistake in simply reading the prompt can mess up your whole essay very quickly, so it is important to make sure it is well understood. While preparing to respond, you can write small notes or sentences that outline the focus of what your essay will be focused on. This bringing me to my second point, planning.
Many find planning helpful but it really is what you are most comfortable with. I personally know that if I plan I have a high chance of "over planning" and will definitely not have enough time to complete the essay. Which is why its important to plan (if you will) concisely and in a way that will be easily accessed when writing. Next, make sure to answer the prompt! This is one of, if not the most important part of the essay. While yes having structure and grammar is crucial, the prompt wouldn't be there if you didn't have to answer it so make sure you do. In addition, make sure to answer the prompt in an understandable manner, having correct grammar, and essay components is also crucial. While it depends on the essay, most need an Intro paragraph with a capturing hook and thesis statement, 3 body paragraphs and a concluding statement. 
That stated you need to make sure each of these components can e completed within your given amount of time, which is why it is important to practice writing them in a timed setting. 

Ehh this was kinda long, I hope it helps!. Good luck on any exams!