Adam made love to his wife Eve, and she became pregnant and gave birth to Cain. She said, "With the help of the LORD I have brought forth a man." 2 Later she gave birth to his brother Abel. Now Abel kept flocks, and Cain worked the soil. 3 In the course of time Cain brought some of the fruits of the soil as an offering to the LORD. 4 But Abel also brought an offering—fat portions from some of the firstborn of his flock. The LORD looked with favor on Abel and his offering, 5 but on Cain and his offering he did not look with favor. So Cain was very angry, and his face was downcast.
Eve's words in verse 1 are heard by Ross (1988: 156) as expressing hope and faith in God's ongoing provision, but Waltke (2003: 96) claims that they reveal an underlying poor theology of divine sovereignty that borders on the synergistic. Ross gives his most generous nod to the first clause, while Waltke focuses on the second.
The brothers' career choices are considered fairly immaterial by Waltke (2003:97), but Ross claims that there is the barest hint in the text that Cain's closer connection with the soil and with plants puts him more dangerously close to the substance (fruit of a tree) that was the formal cause of the expulsion from Eden (188:156).
Ross and Waltke agree that the material offerings in themselves were equivalent: Ross does not even really address the issue of plant vs. meat, moving straight to the clear problem in Cain's heart, though Waltke takes the time to acknowledge that Gerhard von Rad suggests that "the sacrifice of blood was more pleasing to Yahweh". Against both Ross and Waltke, there is the colourful James Jordan, who argues forcefully (1985:159) that the principle of substitution had been articulated by Yahweh already (the provision of animal-skin coverings at the expulsion) and that Cain should have raised or purchased a lamb for the purpose of this offering.
This third point does appear to have more theological stuff packed into it - it's a question of how tightly and richly you want to weave your biblical theology. But on the first two differences of opinion I am at a loss to explain the reasons for the choices made.
James Jordan, Judges: God's War Against Humanism (Tyler, TX: Geneva Min., 1985).
Allen P. Ross, Creation and Blessing: A Guide to the Study and Exposition of Genesis (Grand Rapids: Baker, 1988).
Bruce K. Waltke, Genesis: A Commentary (Grand Rapids: Zondervan, 2001).