According to Genesis 4:22, Cain's descendents were the the first people on earth "who forged all kinds of tools out of bronze and iron". They opened up the idea of warfare and strife. And because of Cain's original sin (killing his brother, Abel), it can be determined that Cain's kids didn't live wonderful, blessed lives in God's favor.
On the other hand, Seth's descendents were quite different. Pay attention to Genesis 4:26, "Seth also had a son, and he named him Enosh. At that time men began to call on the name of the LORD."
There is a clearly an immediate difference. Cain's descendents will be wanderers (Gen. 4:14) and begin to make tools possibly for war, while Seth's descendents begin calling on the name of the Lord.