I always do reverse grip pull ups, i find front grip so much harder. BUT i know that you're supposed to do both because they both work different muscles.
So, which muscles do each type work?
Many muscles are involved in pullups (over-grip) and chin-ups (undergrip), but the main muscles are the Latissimus dorsi muscles on either side of the middle back-not the arms.
If you are flexing your arms (as in both a pull-up or chin-up), you are always working your biceps, regardless of grip. Overgrip tends to require more strength across the shoulders. Undergrip requires more strength in the forearms, and if you have strong forearms naturally, it would explain why chin-ups are easier for you.