My advice to you is to pick one resource and stick to it. If you need additional resources you can add those on later.

The idea that the Manhattan set or for that matter the Veritas Prep complete set of materials is not enough to take someone over the 700 mark is certainly not true. Most people would be better served by using one set of materials and trying to develop a consistent approach.

Let me specifically argue against over-doing it on Critical Reasoning. See how you are naturally at this section of the test - it is the most natural type for many, many people. You can then use some basic strategy to help you focus. The Powerscore CR book is based on the LSAT and adapted to the GMAT. If you want to learn formal logic for the LSAT that is the way to go but the GMAT is a different sort of test.

What you do want to have are challenging questions - in addition to the strategy books. For that you will want official questions (Official Guide 13th edition, GMAT Prep Pack 1 add-on, Verbal Review and Quant Review 2nd editions, etc).

You can supplement this with online materials such as the free Veritas Question Bank https://www.veritasprep.com/gmat-question-bank/ The Question bank gives you questions at a range of difficulties including many questions that are above the 90th percentile (700). Over 2 million questions have been attempting as of Jan 1 2014 and all of those responses have gone into rating and improving those questions.

Finally, let me put in a plug for Beat the GMAT. If you find that your chosen study materials are not sufficient in a particular area - such as Data Sufficiency - you can go to the "GMAT/MBA library tab" and search for articles on Data Sufficiency. You will find certain authors that you relate to better and you can search for articles by that author. You may find this more helpful than additional textbooks.

And of course you can always come here and ask a question and get expert advice as well...

Best of Luck!