Web Design Firm or Consultant? Which do you recommend?

Hello everyone. I am not sure this is the correct venue for a discussion of this nature, but I thought I should give it a try at the very least.

We are a small company with an existing website that was designed by a marketing firm several years ago. This site has not been maintained throughout the years and now needs a complete overhaul. I have convinced the powers-that-be to either hire a Web Design Firm, or contract a Web Consultant to complete this overhaul.

I have partial technical specifications complete, and I suspect our solution to complete those specs, develop the site and train someone in Marketing to maintain areas that are subject to updates. I also want the solution to bring ideas to the table and help us improve upon our specifications. I also want to own the code when the project is done, and I want the artwork to be available in layered files for updates as necessary. Our current artwork is all flat file so I cannot even change a title.

The question is... should I hire a firm, or a consultant? I have some thoughts of my own, but I thought I should best prevail upon the experts!