Watch any time a "hot" artist tries to be a writer to answer this question. McFarlane immediately comes to mind. Thinking of some of who I consider to be the greatest Marvel artists (Ditko, Kirby, Perez, the Romitas, the Buscemas) didn't write their greatest works themselves (I'm sure you could come up with occasions when they have, but generally, they don't). Two that jump out who did start writing successfully were Byrne and Miller. But most of Byrne's success came when he wrote AND drew (a lot of the stuff that he just writes is just average). Sometimes these guys don't realize that co-plotting is not the same as writing.
If you pushed me on who I would like on a title: an a-list writer, or an a-list artist, I would choose the artist, because to me the visual component takes priority. I think that a good artist can put a shine on average scripts and really make them something.(That only goes for average scripts. When they start getting too bad, well, there's only so much you can shine a turd. Marvel pairs Claremont with some really good artists, but the end product is always the same: too convoluted) On the other hand, the best script in the world can really be mangled by someone who doesn't know ho to tell a story visually. Every time a "normal" artist fills in on X-Men for Bachalo/Ramos, the story actually begins to make sense. Which tells me that the story is solid.