Actually, Powerprep algorithm is the same. Hence the way it selects newer questions is the same that one would encounter on the actual test. Therfore, for me, PP gives the best 'simulation'. You need to give it before you start the OG though (At least one test )
I thought I would try to convince somebody to do a favor for the humanity (Anupag, Tzec, anybody?): I had an idea to finally remove the mystery from the GMAT algorithm and I need some help with that. Basically, just take the test several times. If anybody is interested to help, I will appreiate a message. You need to have taken the PP tests already; otherwise it will be bad for you.
I hope to hear from you,