Knoppix is VERY EASY to install. Debian Sarge is pretty easy to install, but it involves more choices and takes longer. Plus, you can actually use Knoppix while installing it.
The biggest tradeoff between the two is:
Pros: Easy to install, and a LOT of useful software already configured
Cons: Impossible to maintain. You'll have to scrap your install sooner or later, because Knoppix just can't take a full dist-upgrade without breaking.
Pros: Good for the long haul; maintainable more or less indefinitely with regular updates
Cons: Not quite as easy to install, and a LOT of useful software is NOT installed or configured.
I recommend a Knoppix variant for a newbie, but not necessarily Knoppix itself. Mepis and Kanotix seem to be the most popular Knoppix variants right now.
I recommend Debian Sarge for after you get sick of not being current enough to install new software. By then, it's time to scrap your Knoppix variant install in favor of something more maintainable. You'll have a bit of a learning curve installing/configuring software which you've gotten used to, but this is a lot easier than jumping in at the deep end from the start!