I am currently struggling with a method to raise my rate with a current client and your comment has crystallized the problem I've been having in writing the email, but not the way you think.
Another way of differentiation is to take stock of what you provide over and above basic work. That is, not to differentiate from any perceived competitors, but from what the client thinks you're doing (their perception of your role to them). If they think you're just doing web development, but also rely on or defer to you for system administration, UX/design, content, and/or hosting decisions, et al, those are all things that comprise your value. This is a roundabout of repeating tptacek's elements-of-value lists linked elsewhere in these threads.
Another way of differentiation is to take stock of what you provide over and above basic work. That is, not to differentiate from any perceived competitors, but from what the client thinks you're doing (their perception of your role to them). If they think you're just doing web development, but also rely on or defer to you for system administration, UX/design, content, and/or hosting decisions, et al, those are all things that comprise your value. This is a roundabout of repeating tptacek's elements-of-value lists linked elsewhere in these threads.