Reddit and HackerNews x-posting: Who gets there first?

I've often noticed a link get to the top page of r/programming or r/technology, only to see it pop up on Hackernews a few hours later, and vice versa. I've hadn't really figured out a discernible pattern, does anyone have any analytics on when links get submitted to Reddit vs. Hackernews? Or any theories on which site gets a link first?