Loren,
The general perception is that KAPLAN is much harder. While I have not given my GMAT yet, I can tell you that I did way better in Princeton, GMATPrep and ETS paper tests than I did with Kaplan.
I suggest you do the GMATPrep diagnostics (free from
https://www.mba.com) and see how you do, and then use that as a basis. BTW Kaplan quant is pretty good for toughening you up just don't their scores as a predictor of your performance.
Good luck.