I would personally expect the workshops to come after the main event. If the main day's presentations give a broad overview of the subject matter, then the workshops can be accessed for more specific information or dialogue in a smaller group. - Sven Latham

If you want people to come to the event too, before. Otherwise they are stressed out. Also it will give people a reason to stay - having it after means "when all the fun is already over and everbody else has gone home" (aka detention like) - Nicole Simon

I don't know if there's any empirical evidence either way, but I think you would usually expect less people at a workshop than at a conference (which is probably better as it's more one-on-one). Before the conference: attendees will probably have had a more in-depth view of the material presented, and would get less from the conference. After the conference and (especially if the material was flexible enough - maybe an "un-workshop") you could tailor the content to what was of most interest/benefit - Ric Hayman