This is my opnion based on my own personal experience. I recently took up my GMAT recently for the 3rd time, and improved my score by 60 points, from 600 to 720.
Only if you feel that your GMAT score will be a detterent to your application, go ahead and take up GMAT Prep coaching. Youve tried to go through the journey thrice yourself, and there are some lingering weaknesses. Join any GMATPREP institute, and get over them. Do not take any more chances on going through the grind yourself.
Sometimes we carry wrong concepts in SC and Verbal. These are difficult to correct unless the underlying ideas are understood.
Dont worry about money now. You already spent 750 $ on your GMAT. You may as well spend a little more money and get through your GMAT confidently once and for all.
I will not recommend the self study again. Not unless your extremely confident that your concepts are solid enough to take the test , and you didnt do well earlier because of other underlying factors, that are not linked to your understanding of the concept.
I would recommend Knewton as well . Somehow hesitated from mentioning any name, else people might think Im a Knewton Sales rep lurking around on Gmatclub. But on a personal experience, basis they are fabulous.
Knewton seems like a good option for re-takers with their 50-point improvement guarantee (or otherwise money back).