Although not an expert, I do see many similarities on both these currents' focus on man himself, aboloshing all idols and taking control of his fate. Are there any real differences between these two philosophies?
Existentialism and epicureanism have at least one thing in common: they are both dealing with lived philosophy rather than mere speculation.
Epicureanism asks how we can live with the most joy and the least suffering--very Buddhist in some ways. Living in harmony with the inherent structure of the universe is what brings about the serenity and peace that ground happiness; living in opposition to the 'laws' of that universe bring about suffering. Pain, though inevitable, can be alleviated through right living or through our attitude toward the inevitable. It's not surprising that much of the history of medicine grew from the Epicureans.
Existentialism also looks at how one may best live but with an emphasis on authenticity, which may mean accepting a meaningless cosmos and, therefore, a meaningless life except insofar as one creates meaning by his or her contribution to life. We cannot escape the actuality of the factors that constitute the present moment, like Sisyphus pushing his rock in Camus mythic analysis. But even knowing that all of our lives lead to a meaningless end gives us freedom and integrity. Such integrity can create a fulness from which we derive satisfaction.
So, though in some ways similar, the two world views have fundamental differences: particularly in epicureanism's apriori acceptance of an ordered world and existentialism's denial that we can ever know the ordered world or even whether there is an order and meaning whatsoever.
I hope that helps.