I don't think the two are at all mutually exclusive--in fact, they play very nicely together (see [1]). Bootstrap is mainly a library to help with layout stuff, while AngularJS provides a way to structure the logic and bind it to the DOM. They're completely different libraries, so learn whichever is more applicable to what you're doing (or better yet, learn both).

[1]: http://angular-ui.github.io/bootstrap