There are great differences between the two in the actual writing, though as you indicate the planning stages are nearly identical, at least in concept.

I'm actually finding that writing a lengthy work of fiction is far easier for me than writing a substantially large piece of software. Novels, in other words, are easier for me to write than major software projects. I seem, at the moment, to have quite a bit more skill for writing fiction than for programming. The outlining of the novel and software design process for the program, however, are (again) very similar, and I seem about as good at both activities.

That's my take, anyway.