It depends upon the function of your site and how much functionality it provides.
- UX, no question, it’s Blink in Seattle, and has been for years now. UX and IA, information Architecture, are slightly different functions, with UX for functionality and IA for content and organization. Frankly, these services are bordering on a science today. Their primary limit is the technology efforts you are willing to pay for.
- UI Design. The dirty secret is that it’s not the firm, it’s the individual designer that matters. Design is very much like fashion. Large firms are highly constrained by branding standards so there is very little design to be done. Firms that do not have standards can be more adventurous and trendy with design. Do no overdo it with design unless all you are selling is aesthetics.
- Marketers are generally best at empirical measurement of design, content, messaging, and positioning approaches for target audiences – which matters farm more than design.
- Creative Directors and Art Directors at larger firms are far better providers of those services. In general it’s the creative director you’re paying for at the agency.
Bet solution (always), is to make screenshots of the different features of the sites of your top 20 competitors in the field (or 50 if you have the energy), and not from your area, but from everywhere, and pick the best features from all of them. And pick the best designs. The purpose of doing this is to educate you, more so than your agency, because this is what your agency will do, and then have to educate you.
Then pay for the Tech or UX or IA or Messaging and Positioning(‘creative’) or Design, or Marketing and Media Buying – or any combination of those that you need.
(I tend to build companies that are heavy on the back end of that (tech, ux, ia, because I traditionally serve the very big, consumer tech companies where creative is limited but message, content, and usability are complicated – but other firms tend to build companies that focus on messaging, positioning, and design. And this is pretty common in the industry. Design small, Tech big.
In general, organizations pay for the size agency they can afford. Any agency with more than fifty full time people is probably legit. It’s easy to find independent designers – they are everywhere, and it’s easy to understand their work. It’s not hard to find UX and IA, but it’s hard to judge their work. It’s hard to find independent creative directors. For marketing and buying it’s best to use an agency. It’s just expensive.