Both restaurants are on our "must" list for every trip.
Yachtsman's has a lovely atmosphere, the steaks are fabulous. Everything on the menu will be familiar to you (most likely), and anything you choose will be beautifully prepared. If you enjoy wine, their list is stellar -- always worthwhile to talk with your CM to get his/her recommendations as well.
Jiko is a world apart. The restaurant is beautiful -- darker and a bit more "mysterious" than the setting at Yachtsman's. There will be options that are pretty familiar and comfortable -- for example, "filet mignon with mac and cheese!" -- but we can tell you that this preparation IS very different from Yachtsman's and worth a special trip. The more unusual offerings are all fabulous -- worthwhile here to talk to your CM about your meal choices as well as wine choices (again, a wonderful wine list)!
As I said, we adore them both. If I were forced to pick just one, I'd pick Jiko because it is unlike anything we can get at home (Boston).