Glad I found this thread—sorry for necromancy.
I think the two philosophies are complementary, not contradictory.
Existentialism, in short, comprises the modern struggle to create values in a world devoid thereof in any longevous, absolute sense. Here's an often-overlooked, but relevant, passage from Nietzsche's
Thus Spoke Zarathustra:
Nietzsche wrote:Of three metamorphoses of the spirit do I tell you: how the spirit becomes a camel, the camel a lion, and the lion at last a child.
Many heavy things are there for the spirit, the strong reverent spirit that would bear much: for the heavy and the heaviest longs its strength. What is heavy? so asks the spirit that would bear much, and then kneels down like the camel, and wants to be well laden. What is the heaviest thing, you heroes? asks the spirit that would bear much, that I may take it upon me and exult in my strength . . . All these heaviest things the spirit that would bear much takes upon itself: like the camel, that, when laden, hastens into the desert, so speeds the spirit into its desert.
But in the loneliest desert happens the second metamorphosis: here the spirit becomes a lion; he will seize his freedom and be master in his own wilderness. Here he seeks his last master: he wants to fight him and his last God; for victory he will struggle with the great dragon. Who is the great dragon which the spirit no longer wants to call Lord and God? "Thou-shalt," is the great dragon called. But the spirit of the lion says, "I will." "Thou-shalt," lies in his path, sparkling with gold- a scale-covered beast; and on each scale glitters a golden "Thou-shalt!" The values of a thousand years glitter on those scales, and thus speaks the mightiest of all dragons: "All values of all things- glitter on me. All value has long been created, and I am all created value. Verily, there shall be no more 'I will'." Thus speaks the dragon.
My brothers, why does the spirit need the lion? Why is the beast of burden, which renounces and is reverent, not enough? To create new values- that, even the lion cannot accomplish: but to create for oneself freedom for new creating- that freedom the might of the lion can seize. To create freedom for oneself, and give a sacred No even to duty: for that, my brothers, the lion is needed . . .
But tell me, my brothers, what the child can do, which even the lion could not do? Why must the preying lion still become a child? The child is innocence and forgetting, a new beginning, a game, a self-rolling wheel, a first movement, a sacred Yes. For the game of creation, my brothers, a sacred Yes is needed: the spirit now wills his own will; the world's outcast now conquers his own world.
Of three metamorphoses of the spirit I have told you: how the spirit became a camel, the camel a lion, and the lion at last a child.
Here we have an entity, the soul, passing through different stages as it resolves its internal contradictions. Sound familiar? That's because Marxism has a similar, dialectical bent. Existentialism, in fact, could lend a personal dialectic to Marxism's impersonal dialectic. These two progressions would run in parallel.
Moreover, like existentialism, Marxism disavows eternal truth as a basis for society, instead preferring to construe culture as an offshoot of its material circumstances. Both philosophies, by their very nature, defy ironclad principles and taxonomies, making them compatible with most any philosophy that does the same.
As I see it, existentialism would help bring a personal character to Marxism's collective struggle for salvation. Marxism, likewise, would help ground existentialism's diagnoses of modern angst in economic theory. Their differing areas of emphasis don't mean they're incompatible.
"Do not wait for the Last Judgment. It takes place every day."
Albert Camus, The Fall