Pullups: palms facing away from you. When you pull your self up, Your elbows are pointed out to the side. They can be performed wide grip, Medium grip or close grip. they can also be done to the front or the rear. ( I prefer front due to less stress to the shoulder girdle)
Chinups: Palms facing you. When you pull up, your elbows are pointed out in front of you. They can only be comfortably performed medium grip or close grip. wider and it stresses the elbow and wrist joints.
They can only be performed to the front.
Due to the position of the elbows during the execution of the excercises, they stress the lats differently.
Which is better? Well that can be debatable. Personnally I do them both. When I do pullups I start wide and work my way closer each set.
When I do chins, I alternate doing them on a straight bar (same as pullup bar) or I'll throw the Close grip rowing bar overtop so palms are facing one another. Again the elbows pointed forward not to the side. I feel that I get a better Lat stretch with these and the chinups then I do with pullups.
I also feel I get better upper lat work (width) with weighted Med/Wide grip pullups.
I say to encorporate them both. examples: alternate workouts, alternate bi weekly, Monthly. split them every workout.