Hiring a supervisor may be worth it only if a/ you know your supervisor well b/ he knows exactly what you needs. If possible he should be someone working with you on your business.
The development of a website requires constant communication between programmer and client: before the project has begun there is communication on the requirements, and several change-approval iterations, during the project development there is communication on possible changes in the requirements due to performance issues, UI recommendations from the programmer, etc. So if the supervisor is not competent enough and does not take the same decisions as you would, then you would anyway have to be involved..so hiring a supervisor becomes useless in this case.
As far as whether a company or a single programmer should be hired: that doesn`t matter, what matters is the experience the person/company has with sites of similar nature. Better find a a few sites you like, find out who built them and contact these people with a request for quote, specifying what exactly you need. Very often companies specialize in specific businesses (e.g sites for media) and do not perform very well on other types of websites...
RE elance: there are some good companies bidding there, but you will hardly notice them as they are just buried among the indian / east european companies..so I wouldn`t advise you to lose time sifting the offers there..
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