I really think you can do more than both - let's throw in backend code there as well. When you know how you can write backend code, front-end code (both on the mobile and the web) and do some devops as well, you become super valuable in the industry and more than that, you'll feel happy.
That being said, it's important focus on one of the technologies at a time. I started out as a backend developer (which is what I'm still the strongest at), but every single day I watched my team's front-end dev's commits and picked up front end development over a period of six months. By the end of 2017, I hope to pick up mobile development as well.