'saranghanda' is a bit literary style. 'saranghae' is an informal expression and you can use 'saranghapnida' or 'saranghaeyo' to older people than you, like parents, teachers, etc.

I know Saranghae, Saranghanda means I love you and both are informal (you can use with close friends and people younger than you) Saranghae, Korean people use it more, it sounds softer and nicer. :)