I picked DK over the Avengers.
Really enjoyed the Avengers for it's entertainment value (also saw Thor, all Iron Man, and Captain America:1st Avenger--great for the back story on the characters(although not necessary)) Many great action and comedy moments in Avengers.
The Dark Knight was more of an 'adult' movie, deeper,darker, and in my opinion made me think more, and kept me on the edge of my seat.
I will not bash anyone for picking either as their favourite of the two. Each person is intitled to their opinion.