Ultimately, you should spend your career doing whatever you enjoy more. You will be working every day for years, so whatever keeps you happy and fulfilled should guide your decision.
I think that the salaries will generally be pretty comparable to each other.
.Net developer jobs tend to be focused more on enterprise software development in larger companies (this is a generalization).
Android development started being more consumer based applications, but there is a large push for enterprise and B2B mobile applications as well. I also feel that in the near future the job market for Android development will be very good.
If you know .Net you should be able to learn JAVA pretty quickly so switching should not be very hard. If you are going to switch, I would learn both Android development AND server side API development skills as well. This will give you more flexibility and more options in the future.