personally i recommend you to do mobile app application. bcoz it has a huge demand so i prefer u to do mobile app.

Depends on your interest. If you see yourself developing apps for phone and that is something you want to do, choose android developer. every website is moving to apps so lots of opportunities there. But there is also a lot of demand for .net and other MS technologies and with good knowledge and experience, it can be a lucrative career path for you.