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).
They serve different purposes: Bootstrap is (mostly) a CSS framework for quickly laying out pages etc, while AngularJS is a Javascript framework for developing front-end application logic. They can be used together pretty effectively for rapid development.
I'm trying to build up a toolset to rapidly build admin sections for services I work on which don't typically have users in the traditional sense.