Mastery Goal vs. Mission for Elementary mathematics

Many of the teachers in my school are interested in using Khan Academy to supplement classroom math instruction and practice this year. In the past, classroom teachers had access to a paid online math program (i-Ready), and many would like something similar that directs students to their individual areas of need. Would they be better off directing their students to the appropriate grade level mission or assigning a mastery goal to students in their classes?

In previous years, my students were the only ones using Khan Academy. I am a specialist teaching grade 3-5 advanced math groups. Since our paid program was very slow paced, my students were instructed to work toward completion of their grade level missions from Khan when their classmates were using the other math program.