It depends what you plan to do, get lots of referrals or make lots of purchases. If you're not sure, you can always join as a CrumbSaver, and if you start getting referrals you can upgrade to a CrumbEarner. (You cannot revert back to a Saver though)
A crumbsaver earns more per purchase, but can only have one generation of referrals. A Earner doesn't earn as high a percentage on his own purchases, but can have 5 generations of referrals.