We offer our clients both SpamAssassin and spam-blocking via several RBLs (realtime block lists).
By default for new clients we turn SpamAssassin on (since it just marks) and we turn RBLs off.
The choices we offer are:
1) SpamAssassin on, RBLs off
2) SpamAssassin on, RBLs on
3) SpamAssassin off, RBLs on
4) SpamAssassin off, RBLs off
If server load were our only concern we'd force our clients to use RBLs to catch up to 99% of the spam, and SpamAssassin to catch 60% or so of the rest.
Not only are RBLs more effective, they take a major load off the server, blocking the spam before it's accepted for delivery, at RCPT time, and the few false positives get directed to a friendly page explaining the problem.
SpamAssassin puts a major load on the server, as it has to scan each incoming email through lots of rules.
And even with multiple and very effective rulesets, we still don't catch over 60% of the spam with SpamAssassin.
However many of our clients want to get all their mail, even suspected spam, so we do offer only SpamAssassin as an option.
If you're running at least Exim version 4.24 I'll be happy to give you our RBL blocking ACL and you can set up the files it requires yourself, or we can offer the entire setup on your system as a service.
For those who don't want to even see spam, we're offering SpamBlocked.net; a complete offsite solution.
For more information on what we're doing and how we're doing it see:
http://www.spamblocked.net/
Jeff