Each step workout has 6 segments, and they are all done on the main workout and then put together at the end.
The premixes take step segments from each of the workouts and mix and match them in different ways.
I just love both of these step workouts, and frequently do the premixes where they mix and match the segments from both. But, I didn't start doing that until I had mastered each of the regular workouts first.
But I think you will find them both fun. Party Rockin' Step 1 has more challenging choreography, but is more steady state. Party Rockin' Step 2 has easier choreography, but more plyometrics.
Both are awesome, and both have fantastic music that perfectly matches the moves. Catch your breath!