The quant guide has completely different problems, so I'd definitely advise working through it if you have the time. It's only like $13 on amazon - a worthwhile investment, to be sure.

It's hard to say how much you'd be missing. I studied only from OG11 and did well, but it's always better to see more problems before your test, assuming you have the time to study them in appropriate detail. The concepts are all the same; what you'll find in the quant review guide is additional application. This is where people typically need work, since the concepts on the GMAT are not terribly difficult - it's the way questions are asked that really confounds people.