if i had to pick one, I would always pick the gmat club challenges
. Take a challenge as often as you can. Work on your weak areas. Take a challenge again. again work on your weak areas. Repeat this cycle till you start hitting 80/90 percentile on regular basis. you are now ready for 49/50+ in Q.
What is your advice for people already hitting 90 percentile.
Anyways that was on lighter note so do not take it seriously.
I have completed almost 95% of GMATClub challenges
and I have also taken 7 CATs from MGMAT. While MGMAT CATs has very small pool of questions, challenges has quite a big pool. If you have to choosen between one of these then definitely recommend challenges as you will get more practice. However, if you can afford then also take MGMAT CATs, because they are equally good if not better.