If you are looking for publishers like Singapore Asian Publications (SAP), EPH, Casco, Victoria, etc , you will go to Popular cos they carry books & assessment books by these publishers. These publishers cater more to our local context. Their assessment books are written in line with Singapore's MOE syllabus & requirements and so when your children use such books, you can be sure what they are learning are relevant and within the required scope. Especially when the syallabus is ever changing and the format of assessment in schools are revolving, you need to ensure that your child gets it right, by buying the latest editions of assesment books. By saying that, I do not mean that old editions are no good, but you need to know what is in and what is out. Then you can supervise your child appropriately.
I have not really bought any assessment books from MPH. They may have but so far, has not been good enough to entice me to buy because of the higher prices in general. Also, I do not have an MPH membership card.
I believe MPH may carry some titles from local publishers and some from other countries. Likewise, for Borders or other major book retailers. If you are looking for a wider variety and spectrum of books, like thinking skills, concept, exploration and activity based books, they may have. Cos, in overseas, their focus is not on so much of routine & drills.
But for me, my objective is to get the child to focus on their mastery of local subject content and therefore, if I go to Popular, I will be able to buy most of the assessment books that meet my needs. I will save my valuable time.
Hope this helps.