Are pull ups or chin ups better for back strength?
They are both great but only if you are doing them right. One good rep is better than loads of rubbish half reps. It is a lot easier to do chin ups without getting much back activation by just using your arms. Watch a video on good form, fitnessfaq and calesthenicsmovement on youtube both would have good videos on it. Just practice, don't expect to get it in the first try or even the first month.
Do 1000 reps of both (over time) and see which one works better for you.