If you have a developer and a designer standing next to each other the person who is going to make more money is the person who asks for more and actually gets it.
If you are making money as a contractor, then you are running a business and that is a skill set which is not mentioned on your poll.
And what about the person who snagged the job in the first place? Hopefully that person makes off with the most money no matter where that person fits into your poll.
There are other skills which are so much more important to making money such as networking, building a client base, branding and the list goes on.
The rest of the skills are easy to get. Anyone can be a designer or developer.