akkifreaky
Heyy ! I took a Manhattan test which is tougher than GMATPrep and got 670.. 3 days later I tuk a GMATprep test and got 620 ..

But I got more questions correct in GMATPres overall and was expecting more.. Wat should I do ? I hav my test in 2 dayss..

Usually GMATprep is supposedly the closest gauge of your likely GMAT performance. I scored within 20-40pts of my prep score and many others have reported the same. However, the Prep exam could be an outlier since there could have been special circumstances. Bear in mind,
MGMAT CATs have tougher Quant sections and easier-to-score Verbal sections.
Look at the questions you got wrong and review your actual experience taking the PrepCAT. Did you do the AWA together as well? Were you flustered in the beginning with tough questions. Did you manage your time properly? Were the more difficult RC questions resulting in more consecutive errors? Look at what you did wrong and ask yourself honestly if you can fix it by D-day.
Most of the time you should go ahead and take the test as just taking the actual exam will be a learning experience and schools won't care about the first score if your 2nd is much better. That's assuming you don't score a 700+ on the first try which you could very well do. Remember, there's supposed to be a roughly 40-50pt variance. So if your true performance is at 680, on a lucky day you could score 730 and on an off day you could score 630.
If you're dead set on a 700 on the first try, you might want to postpone, especially if you can't afford to wait until the next calendar month to retake the test or there aren't open slots early September for you to get a new score before R1 submissions are due. Check the GMAC site before you consider making a decision. 250USD is a hit but unless you are in very stretched financial conditions, that shouldn't even be a consideration.