this is a tough one, as they are both top choices for me! Yachtsman is definitely a more traditional steakhouse, and Tiffins has a more exotic menu. Both have amazing food and incredible service.
I would just kind of double check the menus and see what might be more appealing. I'm going to take the easy way out on this one and say...if you're staying in the Boardwalk area, go to Yachtsman, and if you're making a full day of it at AK, do Satuli for lunch and keep Tiffins for dinner. You really can't go wrong with either choice. The octopus and wagyu beef at Tiffins were some of the best things I've eaten on Disney property.