First, the two skills feed off of each other, so doing one makes you better at the other. However, if I had to choose one, I would choose being a writer who edits. Why? When someone is a writer who edits, they can think like a writer when they go over someone else's copy, and this allows the person to contribute all those things that are editorially correct but weak when it comes to writing. Allow me to offer an example:
"Tom looked past the highway to the east, where the Braidwood Nuclear Generating Station filled the horizon. Any driver cruising down Interstate 57 might overlook the two cooling towers rising above the treeline like so much gray Tupperware. Once Tom had pulled over and stood outside his car, the facility came into full view. All he could do is stare at the 2,000-megawatt nuclear power station filling the view, making the housing development next to it seem irrelevant."This is a description that is, from an editor's perspective, grammatically clean. It offers a description that offers insight and would likely have value to the reader. Most editors would sign off on this. However, a writer would want a little more.
What is more? Well, this kind of description tells us nothing other than what is, and this subject is fertile ground to expand on different facets of the character. This description of the Braidwood station could also describe Tom, which offers the reader more than just an image. It fills in character.
Is Tom technically minded? Does he look at such a place in awe of the technology, and how the engineers who made such a monstrous place could tell him the reactor's weight to the ounce? Is he fascinated by the power generated there? Does the thought of 2,000 megawatts impress him? Does it scare him? Does he think it's crazy to have a housing development right next to a nuclear facility? Does he care? We could learn a lot about Tom just with a few tweaks. Let's take the last sentence and just change the last clause:
"All he could do is stare at the 2,000-megawatt nuclear power station filling the view, making the housing development next to it seem like matchsticks ready to burn."
Those last few words now give us a mood. The reader engages immediately - is this foreshadowing some disaster? Is Tom that scared of nuclear power? How will this affect him?
An editor will rarely offer such a note. A writer who edits is far more likely to call out that spot as an opportunity, and offer a chance to make the writing that much better.
Asked and answered.