I'm currently enrolled in Knewton and also have the Manhattan books
I started with Manhattan, but my extremely rusty math skills made the Manhattan books
overly challenging for me and I just had a hard time staying focused. So after a month of ineffective study, I finally decided to purchase Knewton. I've now completed all of the Knewton quant lessons. (My verbal has always been strong, so I skipped those lessons). Knewton kept me engaged and thoroughly explained key concepts. I think it's an invaluable tool for building foundational content knowledge.
Last week, after having completed Knewton, I returned to the MGMAT guides
. I now understand so much more of what I'm reading. My quant score is currently in the low 40's and I want to bring it to the upper 40's. I believe MGMAT will help me with that.
In terms of your study plan, I don't think that Knewton covers anything that Manhattan doesn't. However, I feel that Knewton is better for targeting your weak areas, because it feels like someone is actually talking to you and teaching you. MGMAT is better for taking your score to the next level.
Lastly, the MGMAT CATs provide so much more information to continue to refine your strengths and weaknesses. Knewton has a 'Concept Queue,' which I think is very messy, and it's hard to pinpoint deficiencies in content knowledge or timing.
Advice, Musings, and Experiences from a member of the HBS Class of 2016