Why not download both and try them? Since they don't touch your current installation you have nothing to lose.
http://www.linuxquestions.org/questi...readid=301137#
In a nutshell, KNOPPIX is based on Debian and has about as much software as you can stuff on a CD with compression. Slax is based on Slackware and will fit on a mini-CD. Two other live CDs you might look at are DamnSmallLinux (which introduced my to the Sylpheed mail client) and PuppyLinux which lets you save your current session back to the CD.
Dennisk