I would go with an agreement if you're in your early stages. Mainly because if things don't work out with all the founders, it's much stickier to legally change things (assuming you're all board members).
IMHO, when you're in your early ideation stage, stay as lean & agile as possible.
I found a couple of articles for you that are much more detailed to your question and hopefully give you something to ponder.
Staying Agile in early stages of startup : http://blog.simeonov.com/2010/02/22/startup-founder-agreements/
Pros/Cons of C, S, etc corporations : http://www.inc.com/guides/starting-a-c-corp.html
Hope that helps.