You comparison-shop for other products or services, and a sign is no different. There are several initial approaches, including reviewing examples of their work, speaking with their references, and doing your research.
Then, consider as you would with any vendor:
the quality of the signs offered
the sign company's ability to design and deliver a sign that fits your needs
the product knowledge and experience of the sign company's personnel
the price, quality and value offered
the follow-up and ongoing service provided
