States are better for simple UI features that nonetheless
need to change, such as a login screen, etc. Navigational
containers such as ViewStack, TabNavigator, etc. are better for
more significant changes in the UI.
The state model will become easier to understand and
represent for Flex 4, but till then, learn states well enough to
understand how to use them effectively, and search this forum for
posts related to states, and you will better understand when they
should/could be used.