I think it is important to have both, afterall, the question when a person leaves a company is who owns the followers? If the person running a company's only twitter page moves to a new company, would they take the followers? If it is their twitter page then they are more than allowed to. If it is an @companyname then obviously not.

I also wouldn't want one person to have so much power, effectively speaking for the company unless it was the person at the top.