This may or may not be true. Certainly, a great script will never save a good story. And an average script won't do anything to hurt a brilliant story. Personally, I would say you should be great at both. Think about it this way. If you or your agent submits this script to a studio, what is the reader going to think? The script can be good, though the story is bad. They won't get past the first ten pages. However, a story is absolutely fantastic, new, uniques, exciting, clear. But the script is average, so they still only read the first ten pages before passing. Remember, the correct term is a "screenwriter." We write for the screen, not the script. Which means that we have to be doing it better and cleaner than the other guy, 100% of the time. Come up with great stories, but learn HOW to write them, not just WHAT to write.