However, some of the material on these tests overlap. ETS creates the GRE you will see on test day, so it provides the best indicator of your score test day. The tests written by ETS are the best in terms of preparation. However, I realize that not all of you will be starting from scratch with your ETS materials.
Taking a test once, figuring out your score, and then hoping that your brain will avoid the same kinds of mistakes on the test is wishful thinking.
The extent to which this is true depends on how you use GRE books and resources like the ETS POWERPREP practice tests. You may have heard that there is no better way to practice for the GRE than by taking a mock exam, especially one written by ETS (the maker of the GRE). By Chris Lele on Main GRE Books, GRE Resources