Current "metacity" packages have the compositor functionality disabled.
Right now, you cannot enable compositor features in "metacity" without
rebuilding it. When you select "desktop effects", "metacity" gets
replaced
by "compiz" and vice versa.
I'm also a little lost. What really happens when you install compiz and
select the Desktop effects in the Advanced preferences? Is metacity
disabled? In this case, should the metacity package be uninstalled? On
the other hand, what's the difference with enabling the metacity
composite extension without installing compiz?
Cheers,
Emeric