In my opinion mercurial's codebase is much better than bazaar's. It's concise and to the point, uses classes when necessary and functions in other cases, following logic is usually straightforward task and it educates well. On the other hand, bazaar's codebase is a mess of hundreds of classes, relations of which are often hard to understand, they are hard to navigate and IMO it's over-engineered. Even writing a plugin for bzr is hard because there is plenty of docs which usually say nothing substantial and codebase is so hard to navigate.

Disclaimer: contributed to mercurial (and wrote some plugins), wrote few plugins for bzr (it was used in one company I worked for).

P.S. Didn't notice right away, hey, Dirkjan! :)