Postfix has been designed from the ground up with security in mind and has proven to be very secure, where as sendmail has had a lot of security problems over the years. Postfix has also proven to be very fast and is fairly easy to configure. Its configuration also takes extra steps to keep you from setting up an open relay or unsecure MTA. I don't know about the user base for sendmail, but postfix's mailing list is very active and helpful.
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