Debian's package management system is great if you have 24/7 internet connection. It can automatically download and install dependencies.
Knoppix doesn't have Gnome, so I'm having problem making apps work that require Gnome libraries.
If you have broadband, then Debian is the way to go. Check distrowatch for Debian review. You don't need to download all ISOs again, apt-get can automatically download required updates and keep your OS up-to-date.