ClassicWB is, IMHO, a little bloated: lots of extras that you probably don't want, too many tools with the same features, ScalOS as a desktop replacement,...
BetterWB works like the usual service pack you're used to find in other OS, it updates lots of things and installs some nice tools. It also has a "new" and revamped look for the classic WB 3.1. Having no extras, it's lighter than ClassicWB.
All in all, ClassicWB is a great all-in-one package; but if you want the basics and build your own system from there, try BetterWB.
