Both look pretty good. MK was a hit until I saw those atrocious shoes; and I love Ashley's outfit but the arm length is... Well, we all know they'll always love something a bit ill-fitting Her shoes are really great!
Ashley at Oscar de la Renta just seems random. I think the last Oscar outfit she wore was that really oversized shirt and skirt ensemble where her boobs were popping out. I think that was, like, seven years ago.
EDIT: Scrap that, she and Mary-Kate wore Oscar at the MET Gala one year.