Assigning group roles, versus letting the kids decide

While planning a group project, I've been reflecting on group practice. And, I suppose, procrastinating those Xmass dinner preparations :)

I'm considering -- instead of assigning roles (image layout, head researcher, assistant researcher), I'd like to gve a list of the necessary tasks, and letting the group decide, who will do what. Students doing the most research, will get the most credit. What do you guys think?