Here is my own humble opinion of doing each Imax with no modifications on an 8" step:
Hard intervals of Imax I:
#2 (64 airborn jumps), #4 (plyo jacks), #5 (jumps onto the bench and ski hops), #6 (lunges off the step - this one really is hard if you try to make it 'over the soccer ball' like Cathe says), #8 (power scissors), and #10 (jacks, leaps, & tucks)
Hard intervals of Imax 2: #3 (genie hops), #5 (long hop turns, jacks, tucks), #6 (power 3's and straddle taps), #7 (jumps onto the bench, ski hops, and tucks), #9 (straddle hops and plie jacks), #10 (power squats and power L's - because the legs are so tired at this point, this one seems harder, so I included it).
I think that Imax might seem harder because typically, one move is repeated over and over, whereas in Imax II it might seem like less because you're shifting your focus. I also think the aerobic sections of Imax II are a bit harder than Imax I so this might fatigue you more, but they are more fun, so you don't realize how hard you're working. I guess if I had an interval workout scheduled and was tired, I would do Imax 2 and that is only because the time seems to go faster and the work seems a tad bit easier. But Cathe is tricky and this workout is deceptive. I did Imax 1 and Imax 2 this week and was equally wiped out with both!
P.S. I love both of these workouts and the music in each is just great! I am so glad Cathe 'pumped up the volume' in Imax 2!!!