In general, I think that there can be a little bit of risk in using too many different strategy guides: if you overdo it, you'll become overloaded with too many options and ideas, and you might struggle to execute any particular strategy with much accuracy. Whenever possible, keep things simple, and try not to cram your brain with too many different GMAT strategies.
I agree with AbhiJ, though: if you have a particular area of weakness, it's probably wise to consult an additional strategy guide in that area.
And no matter which GMAT strategy guides you prefer, it's always good to make sure that you practice with a good variety of official questions during your studies. No test-prep company does a perfect job of copying the question style of the GMAT, so you'll always want to lean heavily on the OGs for your practice materials. I'm not sure how much you've used the OGs, but it's possible that some extra time with official GMAT questions could have eased some of your discomfort with the GMATPrep software.
Good luck with your studies!
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