UIPanel vs UIWidget container

[ngui]

Just wanted to know the difference and use for both UIPanel and UIWidget container. I understand that a widget is in theory an individual UI element and a panel is a container, and that a UIPanel allows for clipping. But it seems a UIWidget can also be a container, and can in situation be easier to use with regard to hierarchy and alpha.

[kittikun]

I personally try to avoid using UIPanel, unless I need the additional geometry (which is actually pretty rare).

UIPanel will creates more drawcalls and probably a wasteful rebinding of common textures/shaders.
You can actually see the effects with the "Draw Call Tool"

For good practice, I use UIWidget as a container so NGUI can optimize the render calls internally.

[ArenMook]

Indeed. The fewer panels you have, the easier your life will be.

Although I generally suggest having separate windows on their own panels as it makes pushing them back and forward easier, and keeps their widgets' depths from mixing together.