onErrorReturn => use case in the scenario of static default value.

Ex:

 public Mono<ServerResponse> handleRequest(ServerRequest request) {
        return sayHello(request)
          .onErrorReturn("Hello Stranger")
          .flatMap(s -> ServerResponse.ok()
          .contentType(MediaType.TEXT_PLAIN)
          .syncBody(s));
    }

onErrorResume- has multiple use cases

• Compute a dynamic fallback value
• Execute an alternative path with a fallback method -Catch, wrap, and re-throw an error e.g. as a custom business exception

This tutorial has examples for each scenarios.