If you are planning to use the 8
MGMAT Guides after that, I would recommend the
MGMAT book.
Other than that both are good. I can't say one is significantly better or worse (I own both). I felt
MGMAT Math Foundations was more innovative in questions/exercises and structured to fit the 8 guides but I liked the layout and logical structure of the Kaplan one.
P.S. both have a number of typos and mistakes.
Edit: When I typed this up, i thought you were asking
MGMAT Math Foundations vs. Kaplan Math Foundations.
If you are looking to start at the beginning, get the
MGMAT math foundations. The Kaplan Math Workbook is a more advanced and different book, though I like it and have used for my prep (it is instead of the
MGMAT Guides).
Hope this helps.