It's not only specific to the server software, but also on the load on that server - spam percentage, amount of mailboxes, time of the year (around the holidays spam volumes increase dramatically) etc.
in one of the companies I worked for, we had 300 mailboxes and an internal exchange server, with another server running trend micro exchange scanner. The spam amounted up to 94% of incoming mail, and both the exchange server and the antivirus server were overloaded.
I installed a front end using CentOS+Postfix+SpamAssassin, using the following: -
~10 different RBLs
reverse DNS lookups
FuzzyOCR (for the image spam)
Helo response delay of 15 seconds (this one cut off ~98% of the spam; requires a lot of extra RAM on the front end)
After this, the internal spamfilter on Exchange went down to less than 1% of the correspondence.