I find that different kinds of writing don't really compete with one another, except for time. That is, I can work on my novel until my fictional brain is tired, and then find that I'm fresh as a daisy for writing a Wise Bread post. And then, if I still have time, I can write a post for my blog here. But if I'm working on a short story, that uses up my fictional brain just like working on the novel does.

Of course, competition for time is at least as fierce as competition for brain power, so I'm not sure how much difference it makes in practice.