Sap wood is the wood nearer the outside of the tree. It is the "growing" part of the tree and as the name says, is where the "sap" flows. Heartwood is from the center of the tree and is more for support/structure. I don't believe sap flows through heartwood.
As for using it for handles, as long as it's fully dried they are both likely fine. I'd venture a guess that the heartwood is gonna be more dense and perhaps a somewhat harder material.