It's been a while since I've eaten often in either, but used to lunch regularly at both when I worked downtown. I think the food is about the same. The Stevenson location is much smaller, so probably has a smaller selection...but I never really noticed that.
I'd say eat at Stevenson if you want a more intimate setting (though it's plenty busy at lunch during the week, as I recall), or Spear St. if you want the bigger, busier setting.
And to answer with a dish...the XLB are one of my favorite things on the menu at Yank Sing, though pricey. They call them Shanghai Dumplings.