Chocolate ganache brings a rich and smooth finish while chocolate buttercream makes the cake fluffy and soft.
Chocolate ganache has a hint of bitterness and is less sweet whereas chocolate buttercream is much sweeter and creamier.
Chocolate ganache has a bold flavor of chocolate and chocolate buttercream gives the cake a creamy flavor.
Chocolate ganache can be used for glossy finish with bold flavor and chocolate buttercream for decorating the cake with texture.