So, I'm a nerd, and I found "Not So Far" more interesting than Lewis's book. I don't think you need to read Lewis' book to get value out of the rebuttal.

I would suggest reading both at the same time. The rebuttal is organized to follow along with the format of the book just for this purpose.

I read Not So Fast without reading Flash Boys, and enjoyed it a lot. Learned a lot about market microstructure.