Bhagavat Gita is the quintessence of Mahabharata and a sermon by Sri Krishna, while Vishnu Sahasra Nama is a prayer sung by Bheeshma as his parting message to Pandavas. Actually Krishna himself requested Bheeshma to reveal these stotras to Pandavas. Bheeshma surprised, questioned Krishna “why are you choosing me at this stage in my life for this, while you yourself could do this?”
Then the Lord clarified : “Mine is like water of the ocean, which is not potable. When the message is through you, it is like rain from the cloud formed out of water from the ocean. This of course can quench the thirst. Moreover, I am the quintessence of Vedas. Whatever I say would appear as though I am extolling myself. On the other hand, if you, a realised soul,speak about it, it has universal acceptance. “Bhagavat Gita is the root of the tree, while Vishnusahasranama is the fruit”.