Both are amazingly talented musicians. If I was looking for someone to headline an epic performance I'd probably go with Beyonce. If I was wanting someone to collaborate with another artist and write a song for them I'd go for Sia.
That's not to say that either of them are not capable of doing the other. Sia might not choose to perform but she's certainly capable of doing it to a very high standard. And Beyonce is known for her collaborative approach to songwriting, even if they're songs for her to sing. In fact, the two have worked together (there's some crazy internet conspiracy theory about Beyonce kidnapping Sia).
Who is more successful? If you define success as being able to live a life where you can make good art, then comparison is irrelevant. More relevant is the idea that one of the keys to their success is their openness to collaboration with others and the evolution of their ideas.