Using subquery or creating temp table

Test table:

CREATE TABLE t1 ( col1 int, col2 int, ... );

Subquery

SELECT * FROM t1 WHERE col1=2

Is it OK to use this subquery two times in same statement or should temp
table created to prevent subquery
executing twice?

Which is better

SELECT *
(
SELECT * FROM (SELECT * FROM t1 WHERE col1=2) p1 WHERE col2=3
UNION ALL
SELECT * FROM (SELECT * FROM t1 WHERE col1=2) p2 WHERE col2=4
) p3
GROUP BY 1;

or

CREATE TEMP TABLE temp ON COMMIT DROP AS SELECT * FROM t1 WHERE col1=2;

SELECT *
(
SELECT * FROM temp p1 WHERE col2=3
UNION ALL
SELECT * FROM temp p2 WHERE col2=4
) p3
GROUP BY 1

?

In real query select statements above contain several tables and have more
sophisticated where clauses.
Using PostgreSQL 8.0+

Andrus.