One of the main aspects between the two is RxJava 2.x which is Java 6+ compatible, but Reactor is Java 8+ compatible. If you are going with Spring 5, I urge you to use a Reactor. If you are comfortable with RxJava 2.x, there is no need to migrate to Reactor. Reactor is an implementation of the Reactive Streams Specification, so you can remain agnostic of what the underlying implementation is.