Composed_of vs has_one

Im trying to decide how to model very simple relationships in

My understanding is that as far as database performance goes, using
composed_of will be easier on the database but a little ackward when
working with the object, considering it the composed_of object is frozen
is not an AR class.

With a has_one, you can include the object onto the main object using
:include, where as, with a composed of relationship, you have to use the
composed_of name to get it initialized onto the object.

I guess the main question is, how much more efficient is it to have all
the records already on the table, than to offload them to a has_one
relationship. To me it seems, like it greatly complicates the query
required to retrieve the object.

In conclusion:
has_one: use when you rarely need to include it on the main object
composed_of: use this when you almost always need this object included
with the main object.

Do you guys generally agree?