The machine based cardio programs are sometimes a better choice if you have injuries because there will be less body impact stress on your body. And it really doesn’t matter what piece. My only advice is if you’re going to use machines in the gym, alternate between the different types. Maybe the step mill one day, rower the next, seated recumbent bike position, maybe even a spin class, or jogging on the treadmill. So try to break it up so that you don’t do the same type all the time and give your body different movement patterns to adjust to while preventing repetitive strain.