Sforno on Genesis 4:2/ספורנו על בראשית ד׳:ב׳

1ויהי הבל רועה צאן, Hevel chose this vocation as it required more intelligence and involved one’s mental activities more that farming.

2וקין היה עובד אדמה, as a result of each having his own vocation, they each brought different offerings to G’d.