The Difference Between the Sacrifices of Kayin and Hevel

After Adam and Chavah were evicted from the Garden of Eden we encounter two of their sons, Kayin and Hevel. Eight pesukim later, Hevel is a murder victim, killed by his own brother. Prior to Kayin's murderous act we are privy to another incident involving the brothers when they both offered korbanot to Hashem. The Torah tells us (Breishit 4:3-5),

ויהי מקץ ימים ויבא קין מפרי האדמה מנחה לה'. והבל הביא גם הוא מבכורות צאנו מחלבהן וישע ה' אל הבל ואל מנחתו. ואל קין ואל מנחתו לא שעה ויחר לקין מאד ויפלו פניו
After a period of time, Cain brought an offering to Hashem of the fruit of the ground. And as for Abel, he also brought of the firslings of his flock and from their choicest. Hashem turned to Abel and to his offering, but to Cain and to his offering He did not turn. This angered Cain exceedingly, and his countenance fell.

Why did Hashem accept Hevel's sacrifice but reject Kayin's? In fact, it would seem more logical for Hashem to have accepted Kayin's - Kayin was the first one to bring a korban; only once Kayin offered his sacrifice did Hevel offer his. Kayin took the initiative, while Hevel simply mimicked his brother!
Rashi famously comments about Hevel's sacrifice:

מפרי האדמה - מן הגרוע, ויש אגדה שאומרת זרע פשתן היה
From the inferior and there is an aggadah (legend) that says it was a flax seed

Rashi says that Kayin's sacrifice was from inferior fruits. According to Rashi this is peshat, the simple understanding of the pasuk. He then quotes a Midrash, which he says is not peshat, which claims that he offered a mere flax seed as his korban. How can Rashi claim that the simple explanation of the pasuk is that Kayin offered inferior fruits? What sort of indication is there from the text that this was the case?
Nechama Leibowitz offers a beautiful insight into Rashi's explanation of peshat. She writes (Iyunim B'Sefer Breishit pp. 29),

לימדנו רש”י לשים לב להקבלה ולאי-הקבלה בפסוקים. בהבל נאמר “מבכורות צאנו”, מן המובחר, ממיטב. אצל קין לא מצינו מקביל לו, אלא “מפרי האדמה” שלא מן המובחר. והשני אצל הבל נאמר “מבכורות צאנו” - משלו, יש כאן נתינת עצמו, הזדהות. אצל קין “מפרי האדמה”, אין כאן שום עניין אישי, אין הנתינה ממלאת מקום עצמו.
Rashi teaches us to pay careful attention to parallels and the lack thereof in verses. Regrading Abel it says "of the firstborn of his flock" - implying from the choicest. Regarding Cain we do not find a parallel, rather "of the fruit of the ground" - that they were not of the choicest. Secondly regarding Abel it is stated: From the first of his flock" - his very own, while Cain brought "of the fruit of the ground." Cain made no individual sacrifice and did not go out of his way to select the best of his personal possessions.

Rashi noticed the subtleties within the pesukim, which show us the inferior nature of Kayin's sacrifice. The Ramban writes that the purpose of animal sacrifice is for the animal to take the place of the person. In reality the person bringing the sacrifice wishes that he could offer himself, but knows that God has outlawed such practice, so he offers an animal in his stead. Hevel understood this, so he offered the best, the choicest of his flock, while Kayin offered the worst; Hevel made a sacrifice with his sacrifice, giving up his valuable possessions, while Hevel simply offered fruit of the earth, possibly as little as a flax seed.