I would say that they are both. Strong clearly believes in the superiority of the United States as a country. However, his belief is driven at least in part by ideas about race. Strong believed very deeply in the idea that the Anglo-Saxon race was superior to all others. For this reason, he felt that countries populated by people of that race were superior to others and he felt that the United States had the best strain of Anglo-Saxons because they had been mixed with other races (by which he means other kinds of white people). This is both nationalistic and racist.
A person who can say the following is clearly both nationalist and racist:
Then this race of unequaled energy, with all the majesty of numbers and the might of wealth behind it-the representative, let us hope, of the largest liberty, the purest Christianity, the highest civilization-having developed peculiarly aggressive traits calculated to impress its institutions upon mankind, will spread itself over the earth.