While Gringo and Yanqui refers to the same thing, the first one in Argentina could be labeled as a pejorative term. Not by everyone of course, but Yanqui has become the most popular option such as Porteño. The word itself doesn't express an opinion on his own, is just used to refer to US Population as "American" isn't especified enough and "Estadounidense" is not really used. Gringo may have a negative connotation by some folks, such as the way someone could pronounce "Boliviano" as an insult. Don't be alarmed if someone calls you that, it may be used in the best way possible, but at least I associate the word with the mexican folklore of US rejection.