SMTP, mail() and piping directly into Sendmail, which is better?

I'm working on mass mailing list application and doing my research on the part with the actual delivery of the message. I've been using a Perl script called MojoMail which sends out blasts just fine and boasts that SMTP is the fastest way for delivery than with Sendmail using a module called BulkMail. I always understood that SMTP adds an additional layer around the MTA which really slows things down, however only useful if you plan to blast emails across several SMTP servers for a load balancing purposes. So that's a 'pro' for using SMTP. The 'con' is that delivery is not quite as fast. Then with PHP's mail(), folks have been saying that because it opens/closes a connection, looping through mail() adds additional overhead and that the best way would be to pipe into the MTA with popen()/pclose() Now I'm curious which method is really better...Piping into the MTA will still open/close the connection and adds the same amount of overhead as mail() do right? What are your thoughts on SMTP?