when to separate Sitecollections to different web applications

Hi experts :)

we're in the middle of designing a SharePoint architecture for a big enterprise consisting of 11 departments and we're wondering of the best separation methodology in SharePoint, should it be a web application per department or a single web app and multiple site collections? and what are the pros and cons of each approach?.

and is there any Microsoft reference or recommendation?

Thanks in advance

Search and Destroy,,,