I think the answer to this question may show who the greater economic enemy is. So who's worse in theory, who has had a worse impact on political thought, the economy and the world in general?
Freedom has always been the only route to progress.
Keynes is way worse than Marx. Marx at least acknowledges that there are competing interests in society. Keynes, on the other hand, pushes this notion of the god-state, that the state can do anything and solve any problem BECAUSE it is the state. Our kind and wise masters will take it upon themselves to be guardians of our prosperity and freedom. This is the best way to structure society, give all the guns and authority to one institution. We and Marx would agree that Keynes is an apologist for the Bourgeoisie. This is the fundamental problem in political theory. Everyone covers the government. I'd rather live under the people's communism than the elite's slave state.
Theory: Marx
Keynsian theory simply leaves us with chronic boom and bust cycles but with a still farily workable mostly free market economy. Marxist theory leaves us in economic chaos with a total state with an excuse for total violence, and so on, I don't know how you could consider Keynes in the same universe as Marx in terms of destructive theory, should his policies be enacted.
Impact on political thought: Keynes
Although the Marxist scourge was terrible it was at least kept under control. Keynsian economic theory dominated everything for a while and is still quite dominant
Impact on the Economy: Keynes
Keynsian theory is still the dominant thought in then economic actions of almost every single state, Marx comes close though considering that for a while his theory dominated nearly half the world, 15 years ago my answer would have been different, but time has told.
World in General: Marx
The tens of millions killed in the famines of Stalin and Mao, as well as the millions killed in the wars and other conflicts involved with their state, can attest to this fact. Furthermore the cold war, as well as the socialist and communist movements of the late 18 and early 19 hundreds were largely inspired by marxist thought
Hey there, I have a quick question, what are the 70s like?
"You don't need a weatherman to know which way the wind blows"
Bob Dylan
The worst of them all was Mises, am I right or am I right?
Keynes, in many ways, is just an extension of Marx. Economists, during the early part of the 20th century, were quick to embrace the Keynesian under-consumptionist doctrine because they already accepted the Marxian over-production doctrine as an explanation for cyclical crises. It must be noted, though, that Marx had an endogenous explanation for trade cycle fluctuations, while Keynes' framework blames exogenous factors (which is why it was so easily fused with Monetarism).
Also, Marx was logically consistent, while Keynes was not. This is why you have fierce debates between the Post Keynesians and the new Keynesians. Keynes had many faces (not all of them were bad). So here's the answer to your question: Marx.
"If we wish to preserve a free society, it is essential that we recognize that the desirability of a particular object is not sufficient justification for the use of coercion."
"Hey there, I have a quick question, what are the 70s like?"
Couldn't say cuz I wasn't there, but from the looks of the wiki description on it it seems pretty.... GROOVY....
"Keynes, in many ways, is just an extension of Marx."
In my experience the marxist critique of say's law is quite similar to what I think is the keynesian critique. But also, the Marxian view of the business cycle is closer to the Austrian than the mainstream in my experience. But in total marxism was the greatest scourge while on the margin, keynesian ideas are the most dangerous as they lead and are leading, into destruction.
The morons that believe them are worse.
"It is crack-brained meddling by government authorities that aggravates an existing crisis."
- Karl Marx
So I'd say...