Personally I wouldn't go so far as to say writing "trumps" story, just that they both have significant weight.

My justification for this is that it is really easy to come up with a good story and really hard to write well.

My justification is simply that good writing should be good through its entire power spectrum, from individual word choice, to a well-crafted sentence, to an engaging scene, to a meaningful overarching plot. Having one component that's excellent doesn't justify poor performance in others; everything weighs in together. (Of course, there are some authors (Dan Brown springs to mind) whose prose style is just so awful that I can't make it through even a single page, so I have no chance of appreciating the plot.)