I ask myself this question a lot. Because I do both, I find that I can't spend the time dedicated to either one that I would like. I'm sure that if I could chose just one to work on, I would get a lot more done and develop at a much faster pace. However, any time I decide to focus on either writing or drawing, I find myself really wanting to work on the other.
The thing that draws me to writing is that I can fall into the process in a way that I have never been able to with drawing. On a good day, when I sit down with my laptop, I stop seeing words and sentence structures, and instead see the story unfolding.
Drawing can be very involving as well, but I find I get easily irritated with myself if the outcome doesn't turn out how I expected. Perhaps the difference is that with a drawing, I don't want to have to redraw it in order to have it turn out how I expect. With writing, rewrites are an accepted part of the process. It may be a little sad or disappointing to have to rewrite an entire chapter, or three-quarters of a book, but I'm confident that it will come out better in the end. I don't have that confidence with drawing. Part of it might be that mistakes are immediately seen in a drawing while in writing, mistakes can be harder to find unless you know exactly what to look for.
If I could only ever do one again, I would choose writing. For now. Ask again tomorrow and I might say the opposite.