I think that react.js is easier to learn because it has nothing to do with the mobile world.
If you come from web development and you have knowledge of css, a little js and html it will be easier for you to understand the concept of react.js.
Although I did the reverse process: first I learned react native after I had a year and a half of experience with ionic, and because I was excited about the react platform I decided to develop with it also on the web.