For these long aerobic workouts I recommend using the stair machine for just the reasons you state. Steady state will be better training than intermittent.
I have been working through the time crunched mountaineering training plan and have come to the longer aerobic pace hike/workouts. This involves sustaining heart rate in a prescribed zone for the entire length of the activity. Due to living in the flat lands, I have two options for this training, a stair mill machine in the gym which has a rolling staircase that you have to step up on, and real stairs which require ascending to the top of the building, about 250 feet, and then riding the elevator down and repeating.
You have mentioned previously that real stairs are preferable to a stair machine due to the stairs falling away from you and reducing the effort required. I do notice that my sustainable pace on a stair machine is noticeably higher than when climbing real stairs which supports the fact that climbing real stairs is harder than riding a stair machine. The down side to using real stairs is the requirement to stop at the top after a 7-8 minute ascent, and then travel back down to the bottom for 2-3 minutes while my HR drops back down into the recovery zone.
For a workout which requires a steady HR to be maintained for an extended period of time, my inclination is to do this on a machine which allows for an uninterrupted ascent and a steady HR for the prescribed period. I’m not sure though if the harder effort on real stairs may be preferable to machines in all types of workouts. There are “interval” type workouts built into the plan which require sustained work for a short period followed by rest periods in between. These lend themselves nicely to a real stair workout. Any guidance from the community would be appreciated.