COUNT(id) vs. COUNT(*) in MySQL
I know this question is about MySQL, but for what it's worth, count(*) is recommended for Oracle, which goes to show that the answer to this can be database dependent (see comment above from BalusC).
Since a lot of databases (MS-SQL, MySQL) have information schema tables that hold various types of metadata, there are bound to be differences if one syntax is simply looking up a readily-available value, and another is going straight to the table.
At the end of day: try different options, and see what EXPLAIN is telling you is going on behind the scenes.