[link|http://z.iwethey.org/forums/render/content/show?contentid=74588|This] thread perhaps? Darrell cites [link|http://www.paulgraham.com/spam.html|this] article by Paul Graham, but it doesn't have any comparisons.
[link|http://home.dataparty.no/kristian/reviews/bayesian/|This] was apparently cited on Slashdot. It has a couple of comparisions.
[link|http://www.helpmesoft.com/product/comparison.html|This] comparison is from a vendor, but covers a lot of products.
HTH a tiny bit.
Cheers,
Scott.