chins are better for biceps - one of the two best mass exercises for them. they also work the muscles along the spine. pull ups are better for your lats. try chins on arm day, and pull ups on back day - you should feel the difference.
Chin-Ups vs Pull-Ups: What Is The Difference?
In kinesiologese, and throughout this article, pull-ups are chin-ups done with a pronated grip (palms down grip) and chin-up are done with either a semi-supinated (palms facing each other) or with a supinated grip (palms facing you grip). What is the best grip for chins?
There is no such thing as a best grip for performing chins. Empirically speaking, the people I know with the best upper back development use a myriad of grips to recruit as many back muscles as possible. You just have to look at the upper back development of the gymnasts who medalled recently in the still rings event at the Atlanta Olympic Summer Games.
etc etc. link for it is:
Bodybuilding.com - Charles Poliquin - Improving Chin-Up Performance!
hope this helps.