I've been an Umi devotee for a couple years now and firmly believe it's the best Japanese restaurant in town by a pretty substantial margin. Why someone would ever go to Nakama over this is unfathomable.
Highlights include the kumamoto oyster (if you've never tried west coast oysters there's no better place to start), seaweed salad, miso black cod (one of my top 3 dishes in Pittsburgh), and the teriyaki salmon (far and away my favorite dish to ever fall under the often-abominable "teriyaki" moniker). I've never gone for the omakase but I imagine there's very little chance of it sucking.
I also like the intimate space, the chopsticks-on-rock arrangement, and the floor seating (although if you're infirm or extremely awkward maybe go for one of the traditional tables...)