I think the UML statecharts are a much more useful diagrams than sequence diagrams. It can convey a lot more of the behaviour especially if you use the entire 'Harel' formalisms.

Too bad, they aren't that popular to document behaviour, because sequence diagrams really don't convey that much.

For example timeouts are very neatly described in a harel state chart. How will one describe timeout and timeout handling in sequence diagrams.