As far as I know the edp is not discontinued but just hard to come by with limited places selling it.
The edp has that dirty civet in the opening and is more thick and full bodied where the edt is lighter and more easy to wear. As a personal preference I prefer the vintage edt as it's more richer than the current one which feels slightly watered down in comparison. To me the vintage edt is the perfect formulation of Heritage.
Between Heritage and Derby I would go for Heritage, I like Derby but that green herbal spicy leather accord is not to my taste and feels a little dated.