I think SpamKiller may have trapped a slightly higher percentage of spam messages. But the Cons easily outweigh this one Pro.
SpamKiller slows down Outlook. Inconvenient User Interface. Help says set POP3 server to "localhost", which is a pain and didn't improve anything. Spam only visible in the McAfee's UI, not in Outlook.
Since McAfee's SpamKiller was bundled with my AntiVirus upgrade, I decided to give it a try. After a couple of days, I decided to switch back to IHateSpam.