I've never been to either, but I've been to Hanoi for similar food they serve at Vien Dong. And I usually eat my ca kho to (catfish in caramel sauce) with com phan at Pho Bolsa or Thanh My. Not sure what I'm feeling like, where would you go if you had to choose one?
BTW, the guy doesn't care for the usual fish sauce dip so that's why I'm leaning towards Xanh. Anything at Vien Dong that doesn't involve nuoc mam (ok if it's cooked w/in the dish).