MailWasher wins on ease of filter setup and maintenance. I was constantly tweaking SpamWeasel's rules, patterns, and words collections and no matter what I did, a week later some spam scientist would send me something that would fly right thru the filters. Plus, SpamWeasel documentation is fairly lacking...and it's not clear as to how you can set up all the options. If you're real comfortable writing regular expressions and can figure out the syntax that SpamWeasel uses, then it can be a great tool. I found myself spending a great deal more time working on maintaining SpamWeasel than MailWasher.
Give MailWasher a try for a few days. It's saved me many hours every week I've used it.