If you sign up for PrepScolar does it incorporate Khan ?? The PrepS blog about Khan seemed very positive and very similar to PrepScholar. Not sure if worth $400 to do PS.
You can use PrepScholar along with Khan Academy, and in fact some of the tests referred to by PrepScholar are also ones used on Khan.
To see the strengths and weaknesses of Khan Academy, check out: http://blog.prepscholar.com/khan-academy-sat-will-never-be-enough-heres-why