diff's between creations of tables

When creating an incremental and unique id, what are the benefits of using:

CREATE TABLE tablename (colname SERIAL);

instead of :

CREATE SEQUENCE tablename_colname_seq;
CREATE TABLE tablename
    (colname integer DEFAULT nextval('tablename_colname_seq');
CREATE UNIQUE INDEX tablename_colname_key on tablename (colname);

One is easier do delete as a dropdb dbname would do it, but anything else I
should know. Or which one is the general practice, any rules of thumb to
use, etc...