Hope this isn't too late to be helpful.
I've been to both and far prefer dinner. Brunch is too loud and too crazy for my taste; when I was there last year large groups of hungover folks looking for more of the hair of the dog that bit them seemed to be prevalent in the clientele. Also, the menu is somewhat limited at brunch.
We went to dinner early-ish one evening and the service was terrific; the service at brunch is much more hurried and, dare I say, a bit cynical?
I've been to the Beverly Hills Bouchon as well and the dinner experience at the Las Vegas restaurant was much more akin to BH than was brunch. Thus, hands down dinner over brunch for me.