Both are tough. IMAX 3 is more fun for me. I think they are both about the same toughness for me cardio-wise, but the hi/lo in MIC hurts my feet. And my knees always hurt the next day or 2 after MIC. Not with IMAX 3, though. A lot of the impact in IMAX 3 is on the step, which absorbs some of the shock. Its a more comfortable workout to me.
Nan