1. In the 19th century Polish lands were under foreign partition - Poznan was occupied by Prussia. Polish citizens where subjected to the process of germanization (f.e. children were forced to learn in german language) but they organized "secret learning" (Tajne komplety) where polish language, history and literature was taught.
So if your ancestors where Polish - probably they speak Polish and German.
2. If they where Polish citizens they most likely have had more common with Polish culture than German and also they considered themselves as Polish people. I don`t know if you know but till year 1795 Poznań was polish city. After losing their sovereignty Polish people many times tries to reclaim freedom by the uprisings(1806, 1830, 1863). Also between 1807–1815 Księstwo Warszawskie (Duchy of Warsaw) was created after Napoleon`s french army defeated Prussia but Poland fully regained independence after the first world war in 1918. So as you can see Polish people had more common with their own people than German people.
Best regards from Poland.