The big difference being that for Marx the collapse of Say's Law was a contradiction of capitalism that would be overcome by revolution, while Keynes believed it could be manages by state intervention.

Social Democrats are secret vanguard party enthusiasts ftw. Lenin actually was influenced by Keynes... mronline.org/2011/04/23/l...

Lenin and Keynes | MR Online

At first sight no two persons could have been more dissimilar. One was a Cambridge don, with more than one foot in the British government; a supporter of the Liberal Party, staunchly opposed to the Bo...

Any resemblance between either of these and a neoliberal policy like "charter schools are good" or "lets privatize the water supply" is entirely in your head.