Below are two methods commonly used in most php codes for fetch mysql data .
mysql_fetch_array()
mysql_fetch_assoc()
I suppose it depends on your definition of big. From that benchmark you can see that it is only before you perform upwards of 1,000,000 million fetches that you actually get a measurable difference? Is your database that big? If not, then go with what is easier for you to use (which is probably to fetch an associative array).
Another point to consider, if it is so big that there is a measurable difference then I would be getting a way from using functions like that all together. Consider using MySQLi or even better use PDO!