Jiko, 100%. The food there is tremendous, the wine selection is awesome, and the decor is top-notch. We make it a point to go there every trip. Sooo good!
Signature restaurants have a sort of "casual" dress code. They request no swimwear, no torn clothing and no hats or tank tops for men. Shorts can be worn but they request "dress shorts"
Jiko - hands down - better food, better menu, better wine selection, and better atmosphere. The dress code is really non-restrictive so I'm not sure that should be a consideration.