They are both really good companies, and I've met really sharp people at both. Palantir's mission is pretty exciting and their employees are generally quite passionate about it, while with Facebook you can say "I work at Facebook." Both companies will treat you very well and I bet you will have a good experience at both.

While your grandmother may never have heard of Palantir, anyone in Silicon Valley who isn't living under a rock has. If your goal is to experience a more startup-like culture, between the two I would go with Palantir given its size and potential.