i believe you are referring to Sumo Japanese Restaurant, and yes they do/did have buffet and sit down service/menu. used to be kitaro, or something close, until they moved over to Day & Nice's old venue on wattana, a large step up for kitaro, though they quite often did i brick business. not sure if successful over there or not, haven't taken notice, that's a lot of sushi just to pay the rent.
also, not sure if Sumo is still in operation, i would suspect so, it's real close to Fuzzy ken's. if i cruise by tomorrow i will try to take notice, on my way to my sunday roast. if not one else pops in before that.
that thread above should fill you in on the other offerings, i think 3 over by mcd, and have seen them queued up also, not my thing. if sumo did close up, i would suspect oishi and clones would of been the last staw for them.