I have seen many messages regarding OG Vs. the Powerprep. The posts center around the theme that Powerprep has only tests. Yes that is the main valueadd; however there are more than 200 questions (and some mutate on OG questions) in the practice tests section of the Powerprep. My suggestion would be not to miss out on these, since they are in GMAT Online format.
Take the practice questions from Powerprep, read Kaplan guide, give Kaplna Diagnostic, work out Kaplan practice problems, study kaplan guide totally, assimilate. Take Powerprep test 1. Finish OG. Take other tests in between including All practice tests of Kaplan CD. Take Kaplan full length tests 1 and II. Study hard on remaining practice tests of Kaplan. Then top up with Kaplan Test III and after final prep, go for Powerprep II test.