If you had a registration, or a wizard component, total of 5 steps each step has text input etc...etc...
would you do this as a viewstack, or would you use states?
If you are using Flex 4, I would use states.
For simple UI like you describe, just one or two TextInput, states are probably the way to go.
As your views get more complex, you might then lean toward a ViewStack.
In a UI where the views are not trivial and there are many views, and the typical user would not use all the views, then you might use modules, so the initial download size is smaller.
If this post answers your question or helps, please mark it as such. Thanks!