I think the question must be asked differently. You should ask yourself what the cost per result is.
Of course, SEO-wise, a well-written 6k word article is way better than a 500 word one. But what about the involved costs? How much would you need to compile the article? Wouldn't it be better to have 12 500 word articles instead of one with 6k words?
The answer is, as usual, not that clear. You always have to think what you want and optimize your actions according to your goals.
Good luck!