Don't base your decision on potential income. Choose which ever you feel most comfortable with and go form there. Likely hood is that neither will make you rich. But you are more likely to be successful doing what you are passionate about.
Web Design has become a very broad term over the years and in my personal opinion it is commonly misused. But I guess that is neither here or there.
Web Designers are supposed to be the artists of the web. They specialize in HTML, CSS and Javascript, while also having very strong graphical skills. Web Developers on the other hand are the engineers of the web. They specialize in a variety of server side scripting languages, Database Design and commonly have at least basic knowledge of server administration. Many of us have evolved to what I would call a Web Professional. Merging elements of both these careers into one.
Graphic Artists, or Designers on the other hand have a much more defined set of skills, and commonly deal primarily in print media. Graphic Artists should not be mistaken for Animators.
The big difference being that Web people wear multiple hats on a daily basis, while Graphics people do what they do... graphics.