I appreciate the fast development pace of Mendeley and their efforts in cooperating with zotero on the users' side. As they use bits and pieces from Zotero code, I wish Zotero interface can also benefit from some of their ideas, too ...
As such, my preference for a good practice would be the in-program attachment viewing (pdf for the moment) capabilities of Mendeley.
Zotero has always had in-program attachment viewing for files supported by Firefox either natively (text and image files) or via a plugin (PDFs)
Well, the biggest advantage of Z over M is the way it can be embeded into my webbrowser - no additional programmes etc.