Debian style install gives you the closest to debian, not pure debian though as the init scripts and such are different.
This explains the different styles: http://www.knoppix.net/wiki/Knoppix_Installer
Beginner style is quite like debian but runs hardware autoconfig on each bootup and thus recreates some config files.
I've always chosen debian style, OTOH I'm running Kanotix now which again is like knoppix but based purely on the unstable branch of debian whereas Knoppix is a mix between mostly testing, some unstable and IIRC some stable. The mix sometimes makes upgrading Knoppix a PITA so if you're going to reinstall you might as well try kanotix. Only thing is that the current Kanotix release has problems with grub on some computers, not all.