Clutter vs GTK+

[hills <hills go2 pl>]

I wonder when you decide to use Clutter instead of GTK+ for application widgets. Is there a clear guideline for this? For example why user menu seems to be GTK+, but date/calendar applet is Clutter implemented?

The main issue behind this question is that users need to use two different widgets, that look and  behave in different manner but do the same things. It may be confusing. The more functions overview mode have, the more non-GKT+-wigdets it have.