In my experience full time salaried programmers typically make more than designers since agencies assume there are lots of people who desire a "cool design job".
However, as a freelancer there is less of a salary difference in the hourly rate you can charge for either position. The one difference that I have noticed is that its much harder for a designer to sell themselves into higher hourly rates. Most people wrongly believe that design simply means making something pretty and forget about usability, content creation, information architecture, etc. The general population also believes that "design" is easy because they have access to a copy of Photoshop and think they are qualified to be designers. All this opposed to a developer who can "make it work" and has a certain mystique surrounding what they do which in turn makes it easier to get to charge a high rate.
I would like to point out that over the years I have met very very few people who are able to be both good designers and good developers. Frequently a person is good at one and at best mediocre at the other. IMHO the two roles require very different personalities and mind sets as well as a vast array of knowledge. That said there are a lot of different factors required to determine which path to follow. Do you love color, composition and type or do you understand numbers, logical progression etc.
I personally do a lot of design (visual layout, color, typography, content creation, photography, 3d, animation and information architecture) and a little bit of HTML/CSS and flash development (I don't do any back end dev, database, etc). I don't use a blended rate but I rarely find it difficult to get $65-$85/hr freelancing depending on the project. And I know that my employer bills my time at $120/hr.
If at first you don't succeed F1... If that doesn't work try Google!
//// Designer, Developer & Teacher - Interactive, Motion and 3D \\\\
Portfolio at WhenImNotSleeping.com