automatic time/user stamp - rule or trigger?

I have a table that I want to keep track of the user who last modified
it and the timestamp of the modification. Should I use a trigger or a rule?

CREATE TABLE stampedtable (
stampedtableid SERIAL PRIMARY KEY,
updatestamp timestamp NOT NULL DEFAULT now(),
updateuser name NOT NULL DEFAULT current_user,
sometext text
);

I suspect that I want a rule, but all the examples in the documentation
seem to update a separate table and not override (or add) the
insert/update to the timestamp and name columns.

Thanks,
-Neal