To be clear, you perform a chin-up with a supinated grip (palms facing you) and a pull-up with a pronated grip (palms facing away from you).
So what’s the difference between these two movements?
It’s impossible to say that Chin Ups are all biceps and wide grip is all lats.
This simply isn’t the case however you will bias the recruitment of different muscles by varying your grip and hand positions.
So here’s what the evidence we have and logic suggests:
Back Recruitment on Chin Ups and Pull Ups
The wider the grip, the more superior and anterior fibers of the lats are recruited.
Narrower grips tend to recruit more the lower fibres referred to as the illiacus portion of the lats.
Arm Recruirment
The brachialis, brachioradialis, and pronator teres would be more recruited by pronated grips.
In contrast, the supinated grips would bring about greater recruitment of the biceps brachii.
What does this mean for you?
If you’re looking to develop certain muscles more then selecting the grio and hand position to bias these fibres would make sense.
However the key point to remember that all grips will and should play a role in your training.
Developing a well rounded and strong back should be the goal.
So I recommend to vary your grip periodically.
This will allow you to maximise the recruitment of all the muscles involved.