You should learn them both. Each technology is a tool. Every tool does a job and some tools do some jobs better than others.
Django is a very productive framework. It's a complete platform out-of-the-box and has an abundant ecosystem of libraries and frameworks that run on top of it (CMS, ecomm, REST, etc.).
Flask is a small core of a framework: the nuts-and-bolts of web development. It's less prescriptive than Django, which means you bring your own set of complimentary tools (persistence/ORMs, template languages, etc.).
Learning both and evaluating them in the context of your personal development approach, projects, and teams will let you make an informed decision about when to employ one or the other.