The difference isn't worth talking about. Either way it's fast. The stats will vary a fiar bit depending upon your hardware.
EDIT | Since we're on page 2 I'll point out that I was referring to 'select * ' and 'select colname ' as opposed to comparing mysql_num_rows() to count(*)
Because MySQL is pretty fast with selects i don't think a second query will add that much overhead..
Btw, once you start using LIMIT in your select * clause, you can't use the mysql_num_rows anymore anyway..