Chins (supinated grip) allow greater range of motion and biceps are in a strong position so you can work you lats harder because you are less limited by your biceps than you are with Pullups.
Pullups work biceps less because your hands aren't supinated but pronated, Pullups work forearms (like a reverse curl) and hit lats with a shorter range of motion.
I do both because I use pullups mostly for Forearms.