Using econometric analysis on survey data from European-born and European-educated researchers who are internationally mobile after their PhD within Europe or to the United States, we find significant positive effects from international mobility on scientific productivity, as well as several other positive career development effects. European researchers mobile to the United States consistently report stronger positive effects on their scientific productivity and on their career development compared to their peers who are mobile within the EU. A propensity score matching analysis shows that this apparent "U.S. premium", is almost entirely due to the different characteristics of those mobile researchers who move to the US compared to those who move intra-EU. After accounting for this selection, there is no longer any significant difference in reported scientific productivity effects between U.S.-mobile and EU-mobile researchers.