I prefer ReactiveUI: https://github.com/reactiveui/ReactiveUI (though you can use it together with MvvmCross)

It might be an unfair comparison as MvX is a much older project but it felt more like something cobbled together over time than something really thought out. I also really like the reactive approach - your view model describes the relationships between its properties as they change rather than have a morass of interlinked event handlers.

Paul Betts is pretty active within the portable-C# community, with multiple active projects: https://github.com/paulcbetts and has a pretty good blog, where at the moment he's going back through the RxUI docs: http://log.paulbetts.org/