I'd put Hue ahead of the Marble Mountains for sure. My first Hyatt stay in Da Nang I didn't even know it was a tourist site and it was right by the hotel. It is not that great, imo. I'd not put Hue ahead of My Son. Hue is a good deal further away though, a pretty full "day" to go there, tour, and go back to Hoi An. Some like to go there on the last day and fly from Hue, so as to avoid the return to Da Nang.
As for the Saigon comment, if you read older threads here you will find a bit of division, some who prefer Hanoi don't like to visit Saigon, some who prefer Saigon don't like to visit Hanoi. I think both are good as a tourist, while preferring Saigon by far to stay long term. But it seems a minority of frequent visitors like both.