using case to select 'which version to use'

Hi all. I have 2 functions , which returns the same columns, but with
tottaly different logics. I want to wrap it with a third function, who
will receive a boolean argument to decide wich sub-function have to use.

I want to use case, so i dont have to use pl/pgsql.

This is a non-working example:

CREATE OR REPLACE FUNCTION get_oferta(varchar, varchar, varchar, bool,
OUT cod varchar , OUT description varchar)
returns setof record
security definer
as
$$
select * from case $4 when true then
(select * from get_oferta_from_a($1, $2, $3))
else
(select * from get_oferta_from_b($1, $2, $3))
end;
$$ language sql;

Can i use case for a case like this?

Thanks!
Gerardo