I am going to back up the GMAT Challenges here... Why? Cause they are kicking my behind... not only because I am not a math guru, but also because there are so many tricks you learn from those... Those really teach you to focus on little details and not jump into conclusions quickly!!! I was even laughing about this one problem and could not believe what little detail I missed
MGMAT material in the books is excellent, but it sure rattles me when I take their online tests... hurts your confidence like there is no tomorrow

Their challengers are really nothing like GMAT IMO... it takes definitely more than 5 min for me to decide what is the best approach!!! So save your brain power for your big date and practice those little things in the GMAT Club challenges.
I think your level of quant is similar to mine (38-42) and I am trying to take it higher, but this
MGMAT stuff is just going nowhere with me and only kills my confidence!!!
just my 2 cents