Working with Outsystems you will discover the enterprise world, discover RAD tooling for doing Web applications (low code framework) and get a skillset that is usually in demand by big corps, similar to SAP, because one only gets into OutSystems working in such kind of environments.
https://www.featuredcustomers.com/vendor/outsystems
It is not something to learn during long nights and weekends, rather with conferences, projects and certifications.
OutSystems is in business since 2003.
ReactJS, well everyone is doing it, it is a cool kids Facebook framework, and open source.
It boils down to which kind of work environments you want to be on.