Using a freelancer or a web design agency should depend on the type or nature of the website. If the website you need is a personal website or just a small information website then maybe a freelancer would be a right choice. If the website is a commercial/official one then going to a web design agency makes sense as for these sites, there are other factors to be considered besides the look & feel. Thinking in long terms and understanding the hidden costs over the entire life cycle must be a consideration.