I don't know what all the categories are that these cases fit into. My understanding is that "unfounded" is a compact way of expressing the opinion that no crime has been committed and the report is false. The proper category for the vast majority of "he said/she said" complaints is whichever one means "the complainant may be telling the truth but on the available  evidence a conviction is unlikely". Both categories will lead to a lack of further action, but the second gives the police an open file to which they can refer if further evidence, including "similar fact" evidence, crops up.  

"Unfounded" should be a categorization that is used only VERY rarely, as proving beyond a reasonable doubt that the complainant is lying is usually as difficult as proving beyond a reasonable doubt that the complainant is telling the truth.