perform 1 check vs exception when unique_violation

I have to insert rows to table with 95% primary key unique_violation.

I've tested 2 examples below:

1)
BEGIN
INSERT INTO main (name, created) VALUES (i_name, CURRENT_TIMESTAMP
AT TIME ZONE 'GMT');
EXCEPTION WHEN UNIQUE_VIOLATION THEN
RETURN 'error: already exists';
END;
RETURN 'ok: store';

2)
PERFORM 1 FROM main WHERE name = i_name;
IF NOT FOUND THEN
INSERT INTO main (name, created) VALUES (i_name, CURRENT_TIMESTAMP
AT TIME ZONE 'GMT');
RETURN 'ok: stored';
ELSE
RETURN 'error: already exists';
END IF;

The first one performs about 20% slower, have 5 times more disk i/o
write operations.
The second one uses 20% more cpu.
Is it because of raid1 and slow writes?
What is the better solution to fit best performance?
Pg version 8.3, table size will probably grow to 100M rows