In "The Labor Demand is Downward Sloping" George Borjas concluded that the overall effect of immigration to the United States between 1960 and 2000 was to lower average native wages by 3%. Ottaviano and Peri, by contrast, argue that the overall impact of immigration was to raise native wages by 1.8%. What are the two key differences between the Borjas and Ottaviano-Peri models that lead them to different conclusions?