What difference, if any, will it make if my hands are over the bar rather away from me or behind the bar towards me when doing a chin-up?
Does this work different sets of muscles?
Is one more difficult than the other?
Should my workout contain different sets of both ways or is it just a preference thing?
Hands facing away tends to focus on your back and lats. Hands facing toward you emphasizes the biceps.
As far as which to use when... that depends on what you are trying to accomplish with that workout. Obviously, the two still use a lot of the same muscle groups so take that into account if you're going to use both.
In addition to hands facing away and hands facing towards you, you can also work different muscle groups by varying the width of your grip. While a wide grip will focus on your upper/outer lats, a closer grip will target your lower lats.
Check out this resource on Pull-Ups for photos and more tips.
