From my perspective, the first and foremost factor in choosing a web design firm is not price, "customer service" and certainly not what other services are offered by the firm, but instead WHO IS GOING TO BE DOING YOUR DESIGN. In a lot of larger firms (particularly offshore ones), you'll see fabulous designs in the portfolio... but the designer who did them is either out of the price range for your job, doesn't work there, or is unavailable. For my money, I want to see the actual designs that the designer working on my site has done, and even more importantly, I want to speak with him/her on the phone, over IM, or (preferably) in person.