Mercurial is much much quicker. That's not an imperical measurement using a large source tree - it's an anecdotal observation on a tiny one. For example, just typing 'hg' returns in 80ms, 'bzr' takes more than half a second: That's just to dump the help text. Actual commits, logs etc.. appear to take similar order-of-magnitude differences in time.
This shouldn't matter to me; with a small source repository its just a small user experience thing, but for some reason it niggles. Actually I can't help thinking this is an easy fix. Maybe I'll crank up the profiler tonight if I have a spare moment.
On the other hand, afaics mercurial doesn't do directory renames. If I move my source directory structure about I lose the per-file history. That sucks, especially for new projects that haven't quite got their file structures sorted yet.