Hii
If you can't postpone the test date, and are supposed to STUDY 2 guides in 20 days, then I will suggest you either to take off from work for few days and STUDY the
MGMAT SC strategy guide or to increase your study hours.
If you feel that even these are not possible for you, then I shall suggest you to go for Aristotle SC grail(latest addition). Though I am a very big supporter of
MGMAT SC, I would like you not to just READ the guide. Its a Bible and you have to study it.
Aristotle SC Grail is more of a summary of
MGMAT SC but don't expect to get the solid explanations. Just concept and examples.
Hope that helps.