I heard an interesting point regarding the comparison of Heter Mechira to Mechiras Chometz:
For those who hold that heter mechira works, they are comparing it exactly to mechiras chometz, because just like the chometz is sold to a non-Jew and then rebought after Pesach, so too, the land is sold to a non-Jew and then rebought after Shemitta.
However, for those who hold that heter mechira does NOT work, they say that if you sell chometz and the non-Jew doesn't want to sell it back after Pesach, we are more than likely to accept the cash value of what they bought, because you could always buy alternative chametz with that cash. However, with selling the land of Eretz Yisrael, no one in their right mind would ever let it go, even if we were offered a lot of cash. The fact that we would never let it go nullifies the sale to begin with.