I guess as Rabbut said, stay away from wild caught species, but you can't really say some Discus are going to be hardier then others.
If the Discus is a really odd strain, and has been quite imbred in order to get it's colors, then yes, it is going to be more susceptible to stunting and disease.
I have heard that some pigeon blood strains used to more more hardy then regular Discus strains, but I am not sure how true this is. After all, all Discus are Discus. They cannot vary from fish to fish very much.