Phoenix should be faster, at least by default, and a better choice for apps that require concurrency; streaming, real-time collaboration, long-running services… However Django is more established, there are a lot of jobs, books and tutorials.
I think Django is more complete by default, and thus it may be overwhelming at first, it may be worth considering alternatives like Flask, but overall both are easy to understand. I’d suggest you to spend a week with both frameworks building the same project (a simple one), and chose the one that makes you more productive.
