I have both, but I have yet to do Low Impact Circuit. However, previewing it, it looks SUPER fun!
Low Max is really really fun, and intense! I am a relatively new stepper, I consider myself intermediate. I fantasize about the day when I can pop IMAX 2 into the dvd player and go start to finish without falling dead on the floor. Right now it is just too intense for me. Low Max is a great, fun workout for those of us who aren't ready for that level of intensity, but we want a workout that makes you sweat, makes you smile, and gets the job done. Also, Low Max blasts are focused on the lower body which is an added bonus.
Low Impact Circuit appears to be really fun, but I haven't done the workout yet. I know lots of people love it.
I would recommend both.