To add another voice here, the stairmill is a great training tool. I’m a flat lander and as a result I’ve spent countless hours on them training with pack and with body weight. Last year my meniscus was messed up and I could run, but could hike on the stairmill and used it for ALL of the Z2 work as part of the fall training group in preparation for a successful trek of Kilimanjaro. Overall I’m a big fan for those that don’t have regular access to mountains.
This is not a knock on treadmills, but unless you have access to an incline trainer that goes above 15%, my strong preference for simulating mountains is the stairmill. Now if you are doing MTN running or something else, then maybe the treadmill is a better tool, but for Rainier I find the stairmill superior. Although when I first went up Rainier I trained with a treadmill and had a very successful summit. Consistency is very important, as is training for the pack weight.