Question about send email through php script

Hi Folks! I am working on a php system which sends e-mail. The system is ready to go now, but I am asking myself and now all of you what´s the best way to send this emails throug a php script: a) Using the mail() function? b) popen sendmail and writing the messages directly on it? I am thinking about which could be faster and costless for the server processing perspective. Regards Alex