Having re-read your post, I think you may be miss stating your goals. While GMAT prep tests are indeed, the most accurate prediction of your score, you don’t need that for measuring your improvement. Imagine you had a ruler, and then a random stick of unknown size, ensure it is better to use a ruler but if you’re pointing to measure multiple times and you want to compare distances, a random stick would do just as fine. That is because your numbers would be consistent.
So what I would say is that you will probably end up buying both most people end up using at least 10 practice tests throughout the prep time and that’s totally my estimate in my guess. Some people go way beyond that but I feel most used 10 at least. This is because the score of the Manhattan test is consistent from test to test and while it may not be as perfect in estimating your real, GMAT score as GMAT prep, it serves an important purpose when you have a very limited number of official tests.
Posted from my mobile device