Hello Oliver,
Thank you for your post.
If we may, we would like to add to this conversation to help you understand how these products can co-exist, and the benefits of having both products on your system.
MagicSpam is designed to reject/tag spam messages BEFORE they reach the delivery system, knocking the worst out before it uses up bandwidth, I/O or overhead. It does this by working directly in the SMTP layer of your system. This is BEFORE it reaches the content filtering levels, such as SpamAssassin.
MagicSpam allow low overhead and low resource usage on your server, very useful for servers with high amount of users and email traffic.
Considering MagicSpam will block most of spam messages before they even reach your content filtering, (such as SpamAssassin) this greatly reduces the amount of emails your email filters are required to process. However, higher intensive content filtering systems such as SpamAssassin can be useful for detecting leakage from perfectly valid email systems (such as Google, Yahoo, Hotmail) and other fringe cases that do happen to sneak past MagicSpam.
This way you can still use SpamAssassin rules, for content based rules such as say 'porn' which one person may want and another person may not want.
However, there are some things that SpamAssassin does not have access to that MagicSpam does, because it happens at a different stage of processing, while SA has it's own advantages when it comes to content. But content is always changing, and sometimes you have to wait for the rules to catch up.
Normally though, expect that MagicSpam would stop the highest percentage, with the lowest overhead, and can also act as a more 'zero' day protection, even when the content changes.
MagicSpam goes even further as it offers additional features such as the Rate Limiter, BMS Lists, custom policies, searchable logs, more robust statistics and many other features. (PRO versions of MagicSpam offer even more flexibility).
In summary, MagicSpam and SpamAssassin are dealing with spam in different ways, but at the end of the day they complement each other very well, and together they offer a great amount of spam protection for your servers.
We hope that this helps in choosing the right option for setting up the anti-spam protection on your server.