Carroll, I've never made this as a two layer cake, only in a tube pan, so I've
never tried the filling. However, I would think that jam would be sweeter than marmalade, and if so, I wouldn't use it.
By the way, this cake is so good, it stands on its own, so it doesn't really need any adornments. I've served it plain, with a simple orange glaze, and with an orange cream cheese frosting - loved them all.