If you have to make a choice at this point, I too vote for the Rhythmic Step / Interval Max / Maximum Intensity Cardio disc, and if you like THAT and still enjoy the I-Max 3 drills on your HCE disc, then get I-max 3 at a later date.
I personally like I-Max 3 primarily for its blast-only premix; keep in mind I'm an absolute klutz when the choreography gets more complicated than a hopturn across a long step, so I'm all about basic choreo / maximum power moves. Impact issues are individual to each exerciser; I personally have no problem with high impact and/or plyo moves and I'm 47.
Many have mentioned they don't care for the step segment of Maximum Intensity Cardio, which comes about 35 minutes into the warm-up. I too find that step segment to be too moderate for my tastes and capabilities. One thing you can do to really, REALLY maximize the intensity of this one is to do the warm-up and hi/lo in MIC; then quick scoot to the beginning of the actual step/blast segments of Interval Max and do as many of these as you can. Major, major cardio and leg power benefits and bragging rights.
A-Jock