I may be a little biased so take my perspective with a grain of salt ... but I see you already have access to the
GMAT Club tests, so at least I have nothing to gain here and not selling you

I have no beef at all with
MGMAT Advanced Quant and absolutely appreciate that they have come out with this book - it is valuable. What I felt and what I see consistently mentioned by experts is that some of the questions in the book are very hard and sometimes that difficulty is artificial. The questions require 3+ steps to solve them and can be discouraging at times. Some are made hard through multiple calculations that are just time-consuming. There is nothing bad about it - it won't drive your score lower but the general feeling is that GMAT does not make hard questions harder by making you do a lot of calculations or 3 steps. Instead those hard questions tend to require applying multiple principles or analysis that helps you get an answer quickly or thinking in non-traditional ways. Here is an example of such questions:
A fast-growing water lily doubles in size every day. If it takes 30 days for a water lily to cover the entire surface of the pond, how many days would it take for the water lily to cover half the pond?
A. 8.33
B. 12.5
C. 15
D. 20
E. 29
This is one of my favorite ones though not the hardest but this is the type of things
GMAT Club Tests aim to prep you for. If you are shooting for Q51, you will see questions on the test that you have never seen before but this is just a biased opinion. There is nothing wrong with doing both potentially since you have access to the
GMAT Club tests already. So, poke around