One (not entirely unfair) way to categorize the two sites was that
oDesk catered to the cheaper end of the market, whereas eLance was often more expensive and by implication, the quality of freelancers was higher (although not always the case).
Professionals can peddle their services, whether it be supply - chain management or legal advice, more easily and independently too, through sites like Elance -
oDesk and TrustedPeer, which sometimes
cater to big companies.