I would call GMATPrep the most reliable indicator, so yes, I'd say your current scoring range is most likely mid-600s.
There are lots of reasons why your score can fluctuate (starting with the fact that these kinds of tests aren't as precise as most people think!). Timing issues, holes in your foundation, differences in testing conditions (eg, if you skip sections or take longer breaks than allowed on some tests), etc. Read this to see which ones might apply in your case:
https://www.manhattangmat.com/blog/inde ... t-wrong-2/Let's start with the most important issue: 10 days out, you score a 650 on GMATPrep, so assume this is the same range that you will score on the real test. Is that okay with you for a first attempt? If so, you can go ahead and take the test (but assume you'll take it again). It will be valuable experience that you can use as you figure out what you need to change for next time.
Alternatively, you can postpone your test (and maybe you have already done this since you posted here).
Next, if you're going to go ahead and take it, just review for the last week - these two articles can help:
http://www.manhattangmat.com/blog/index ... an-part-1/http://www.manhattangmat.com/blog/index ... ew-part-2/(If you postpone, just save those two articles for later.)
If you decide to postpone, I'll assume that you want to go for that 700+, so give yourself at least another 4 weeks (and 6-8 might be better).
Then, (whether you postpone or take the test and then keep studying), we're going to need to dive into your strengths and weaknesses.
First, read these two articles:
https://www.manhattangmat.com/blog/inde ... lly-tests/http://www.manhattangmat.com/blog/index ... -the-gmat/Then, use the below to analyze your most recent MGMAT CAT (this should take you a minimum of 1 hour):
http://tinyurl.com/analyzeyourcatsFigure out your strengths and weaknesses as well as what you think you should do based on that analysis. Then come back here and tell us; we'll tell you whether we agree and advise you further. (Note: do share an analysis with us, not just the raw data. Part of getting better is developing your ability to analyze your results - figure out what they mean and what you think you should do about them!)