In my experience more than the scheduling technique, the real bottleneck in sending emails is with the mail server.
It's very difficult to track delivery, avoid blacklisting and taking action on error reports.
For announcement newsletters and stuff, CampaignMonitor or MailChimp does a good job. But seems services like MadMimi (
http://madmimi.com) is even better, because of the out of the box Rails support (Here is the plugin -
http://github.com/redsquirrel/mad_mimi_mailer/)
--
LP