What is faster?

the Merge statement

    MERGE INTO table
     USING dual
     ON (rowid = 'some_id')
    WHEN MATCHED THEN
     UPDATE SET colname = 'some_val'
    WHEN NOT MATCHED THEN
     INSERT (rowid, colname)
     VALUES ('some_id', 'some_val')

or

querying a select statement then using an update or insert statement.

    SELECT * FROM table where rowid = 'some_id'

if rowCount == 0

    INSERT INTO table (rowid,colname) VALUES ('some_id','some_val')

else

    UPDATE table SET colname='some_val' WHERE rowid='some_id'