If you have to choose, I'd say web because a lot of sites are moving to progressive web apps as opposed to native mobile apps. The web is getting smarter and more capable. That said, there's also a lot of value to learning mobile development. It really depends on what you want to do or where you work/want to work. This is just my opinion if you're trying to choose one as a focus. There is a lot out there, so you don't even need to limit yourself, just give yourself stretch goals for what you'd like to learn, it's just a matter of where to start.