Hmm, I don't know if Moffat did the more adult episodes, really - darker in that they were scarier, yes, but thematically Moffat tends much more toward morally simplistic and happy endings. "Blink" is scary as hell, but I don't know that it's particularly more mature. RTD did the more adult eps, emotionally speaking - "Midnight" or "The Waters of Mars" were much darker than anything Moffat's written for Who.

In that, I think the situation with the space whale is the most morally dark thing Moffat's written for Who? And that was resolved a lot happier than, say, Pompeii...