chin ups to me bring in to much bicep and not enough back. so with that I'd go with wide grip pull ups. they help to activate not only your lats but a few more smaller muscles that surround the lat. So (IMO) go with wide grip pull ups.
I used to do behind the neck wide grip pullups and they worked pretty good. But now I have switched to wide grip pull ups to the front and it has increased the activation of my lats and have widend me out quite a bit in a short time! Add weight and it's even better! Just make sure to pull your elbows a little behind you to activate your lats the most! IMO, Chinups will develop more of your serratus and not contribute to really widening your back!