Sometimes the difference is a good thing - the noodles are a billion times better and fresher here. My noodles yesterday were hand-tossed from a pile of dough three feet from my table before being tossed in the wok and fried up with some pork, mushrooms, and red sauce. The omnipresent mutton skewers and their curious cumin rub are all but absent in the US so far as I know, apart from a mere shadow at Feng Mao Mutton Kebab in LA. The yogurt here is outrageously tangy and delicious, sold out of stalls in the heat, but somehow not spoiled, straws puncturing the foil tops of their bottles.
Sometimes it's not so good. The baozi here have been disappointing - bitter bread and a droopy filling of sad boiled vegetables. Tales of exotic fruits have proven to be inaccurate, as mushy apricots and rock hard kiwis have been the norm. There is a mystery vegetable that the cooks in the Muslim Quarter of Xi'an put in just about everything - pancakes, buns, soups - that numbs the sides of your tongue, up and down, and is very disconcerting.
In both Korea and China, I've had much more luck with street food than with restaurants, possibly because my preferred method of dining is grazing. I'd much rather wander through a market popping fried quail eggs off the stick in my right hand while gnawing on the carrot-sesame pastry in the other than sit down in a restaurant and try to finish an entire bowl of mutton and wheat noodles in broth with absolutely no help from my stubbornly vegetarian companion. Food trucks in the US do nothing to compare to that grazing experience. The Taste of Chicago comes closer if you ignore the overpricedness of it all.