Example 1

[mikemeister_admin]

I used the Lab result because I felt that the RGB result was too over saturated and looked unrealistic.

[-default]

Very nice - you got a good shadow point which is the main object of this exercise.  The second part of this exercise, picking your favorite color space, is more problematical, and you are correct that Lab gives a more natural result. 

I happen to know this for a fact because I used an Lab adjustment on the original image to create the example image!  Another big clue is that the sky takes on a cyan cast that is a bit unnatural.  That said, all of the exercises have multiple correct answers, and there's nothing really wrong with choosing the RGB version - the point is really to know that you have a choice, and to pick the one that appeals to you.