Variation is the key...do different types. IMO, you should work most on your weakest movement. Alot of people can do pull-ups easily and chins are tough (or vice-versa)...so work on the weaker of the two...but always vary it up.
You can also do them with palms facing (if your gym has that grip), or throw two towels over a horizontal bar, and just grab those and pull yourself up, this is good for your grip, arms, and back.