A bit of both, depending on how old the planet's colony is.
There's no aliens in the Milky Way, so all inhabited planets are descended from earth, give or take some genetic alterations to help the people adapt to the environments.
So you'd have hundreds to thousands of colonists from all walks of life and cultures, and they'd gradually meld together into a distinct society with subcultures. As they spread out across the planet and new nations are formed, the culture would fragment and split into a diverse world of different ideologies.
So a planet could have some unanimous cultural through lines, but not enough to be a monoculture. I hope.