The step mill - you set it and forget it. If you don't keep up, you fall off the bottom.
All the other machines, like rowers, you have to keep focused on keeping up your tempo.
10 days out, I don't feel like doing any cardio. I am probably not going to keep up the same intensity for 45 minutes. On a step mill, I program in the workout, set it for 45 minutes, and damn it, I either do what that machine dictates or I will fall off the bottom.
If you want to get a rower and feel like it will give you the best results, then by all means do it! I'm not into 45 minutes of rowing while prepping for a contest, starving, lacking sleep because I am waking up hungry, and getting on a machine at 4 or 5 a.m., the step mill is my choice.