We went to both on our trip in August. I don't think you can go wrong with either. Yachtsman Steakhouse has a more traditional steakhouse atmosphere, whereas Flying Fish has more creative design and feels a bit dimmer, which I like in a restaurant. That's honestly the biggest difference I can find. I thought they both had interesting and creative food on their menu, and we enjoyed both. (I enjoyed Yachtsman a bit more, but that's because Flying Fish was our family "nice meal" for the trip and the toddler was tired, whereas Yachtsman was a date night.) I might give a slight edge in creative menu to Flying Fish, but very slight. I had seafood at Flying Fish and a steak at Yachtsman, because that seemed right. I'd like to do a date night at Flying Fish in the future.
Don't think that helps much---you won't go wrong. Skip dessert, walk around a bit then go to Ample Hills regardless of which you pick.
Bruce