I prefer chin ups to pull ups, but generally do both, every workout.
I have to use an assisted pull up machine (or, occaisonally pulldowns) due to trying to be progressive with the load. (I weight 270lb, so trying to pull/chin that weight, plus add weight each w/o is very difficult!)
I generally do 2 sets of 15 reps, then 3 sets of 10 reps, and 5/6 sets of 5 reps. I usually alternate, so the first set is pull ups, the second set if chins, etc..
Great exercise, especially coupled with dips, works the whole upper body
Hope this answers your question
Brix