$_REQUEST, by default, contains the contents of $_GET, $_POST and $_COOKIE.
But it's only a default, which depends on variables_order ; and not sure you want to work with cookies.
If I had to choose, I would probably not use $_REQUEST, and I would choose $_GET or $_POST -- depending on what my application should do (i.e. one or the other, but not both) : generally speaking :
You should use $_GET when someone is requesting data from your application.
And you should use $_POST when someone is pushing (inserting or updating ; or deleting) data to your application.
Either way, there will not be much of a difference about performances : the difference will be negligible, compared to what the rest of your script will do.
But it's only a default, which depends on variables_order ; and not sure you want to work with cookies.
If I had to choose, I would probably not use $_REQUEST, and I would choose $_GET or $_POST -- depending on what my application should do (i.e. one or the other, but not both) : generally speaking :
You should use $_GET when someone is requesting data from your application.
And you should use $_POST when someone is pushing (inserting or updating ; or deleting) data to your application.
Either way, there will not be much of a difference about performances : the difference will be negligible, compared to what the rest of your script will do.