How did nationalism affect Germany and Austria-Hungary differently?

Answer 1

In general, you could say that German nationalism tended to bring the country together, and the nationalism in Austria-Hungary tended to pull them apart. There was a much greater land area, much greater religious and cultural diversity, and a much greater overlap in people identifying with other countries and language differences in Austria-Hungary.

Nationalism of course continued. After the dissolution Austrian Catholics were bonded together by religion, and although many of them were German, they didn't want to join with the protestants. German nationalism in Nazi Germany continued, and may have been a unifying experience for some, but at the expense of the ostracism and dehumanization of others.

Answer 2

Nationalism brought the various German states together in a new German Empire, by making the various Germans aspire to a political union to correspond with their ethno-linguistic union. Conversely, nationalism ripped the Austro-Hungarian Empire apart. The Austrian Empire was ruled by the German-speaking Austrians who were a majority in a very small minority in the whole empire. There were far many more Slovenes, Croats, Serbians, Bosniaks, Hungarians, Romanians, Poles, Czechs, and Slovaks than Austrians and these peoples would eventually demand independence on nationalist grounds. When this nationalism was realized at the end of World War I, the Austro-Hungarians Empire ceased to exist.