Math Mammoth Help

I am trying to get something that my DD will like and will retain.  We have used MUS but she isn’t retaining it.  She seems to have it mastered and later I realize that she really doesn’t.  I have finally decided that we need something completely different.  I am looking at Math Mammoth.  She seems to have gaps in her learning/retaining all the back to very basic things.  So, my thought was to get the blue series that is topical and just go through the books.  Some we will progress through quickly and probably skip parts and others will be slower as she (hopefully) fills in those gaps.  I would really like to get her up to grade level by next school year.  I was hoping that this would work.  If that works, I will probably finish out with MM as far as it goes.  This year she is 6th grade, but has not finished MUS Delta.  If you have used it, is the blue series complete enough to use as remedial and then to finish out as a complete curriculum or do I really need to get the light blue series?  They are on sale right now on the Homeschool Buyers Coop and I was trying to decide if I should just purchase the blue series for $60 or the all inclusive series (blue, light blue, golden and green) for $116.50.  If I knew I would be using the light blue series it would save me in the long run to buy all of it.  What is your recommendation on the blue and light blue series and would you buy the all inclusive or just the blue series?