Have never heard of GMAT Quantum. What is it? And also, are you recommending MGMAT for Quant also, or only for verbal?
GMAT Quantum is this site: http://www.gmatquantum.com/
He provides free explanations to all OG problems. He has a premium portion of the site, but I never used it.
I recommend MGMAT for Quant, but quite honestly, it's not fully comprehensive even though it's the best on the market. I supplement A LOT of what I learn with googling/etc. For instance, I think it's Geometry book is painfully simplistic. Number Properties is the best out of the whole thing.
For Verbal, I'm actually naturally good in Verbal (RC > CR > SC). MGMAT is the best, but I feel like I need a different approach for SC. It does give a great overview of grammar topics to be tested, but you have to review the OG SC problems consistently since GMAT grammar is hard to duplicate.
LOL, I swear the SC problems during my GMAT exam were not real English. That was always my weakest link in the GMAT Prep