Both have slick, powerful UIs. Asana has a nice Chrome extension. Producteev seems to have a better Android app. Producteev has a global task list view, which is a major drawback of Asana.
Yeah, one major failing of Asan is that there is no global task list view.
Another is that they have just told me they have no immediate plans to add a widget to their Android app. I think they just don't grok Android. That's a dealbreaker for me.