Sinister, by far.
It's probably my favourite US horror of the last few years, it has a brilliantly creepy atmosphere and the soundtrack is incredibly unsettling, and even though i usually find "jump scares" to be cheap, here they were used very effectively. The only thing i didn't like was the scene with the ghost children following Ethan Hawke, it would have been better if they'd kept them blurred out in the background rather than showing them in focus so that we can see that they're literally just kids with make up on. Aside from that brief scene though it's a great film.
Insidious is good until you actually get a good look at the demon half way through the film, after that it just falls apart and the tension disappears (partly due to the way that it looks like Darth Maul with hilarious little goat feet).
Sinister - 8.5/10
Insidious - 6/10