Data-binding is the most obvious feature of Ractive but there are others - easier event handling, transitions, animations, JavaScript-ish expressions within templates, server-side parsing and HTML rendering, and so on.

On the other hand, Rivets is much smaller, so if you only want the data-binding then it's a fine choice. Personally I really enjoy the freedom of writing mustaches straight into my template, rather than using data-bind attributes everywhere, but it's horses for courses.