Your workout intensity on the treadmill or stair climber influences calories burned.
Both the treadmill and stair climber can be effective weight loss tools as long as you work out at an adequate frequency, duration and intensity. The one that is most efficient for losing weight depends on which one burns the most calories. Because the overall number of calories you'll burn partly depends on how often you exercise, although one may burn more calories than the other, choosing the one you enjoy the most will help you be more consistent with your training and thus lose more weight.
Causes of Weight Loss
Calories burned during your treadmill or stair climber workout are significant in deciphering which activity is best for weight loss because you will lose weight whenever you burn a greater number of calories than you consume. When you're consistent with your workouts, you significantly increase the number of calories you burn and you have a better chance of creating a caloric deficit. When you reach a deficit of 3,500 calories, you'll see a pound of fat loss (See References 1).
Head to Head
Whether the treadmill or stair climber will burn more calories depends on a few factors, including how much you weigh and the intensity of your workout on each machine. According to MayoClinic.com, a 120-pound person will burn about 657 calories in 60-minutes of working out on the stair climber. A person of that same weight will burn just 204 calories if they're walking on a treadmill at 2 mph and about 314 calories if walking at 3.5 mph. If they picked up the pace and used the treadmill to jog at 5 mph, they'd come close to calories burned on a stair climber, burning about 606 calories in 60-minutes. The calories-burned chart provided by MayoClinic.com state that It would take a running pace of 8 mph on the treadmill to burn more calories than the stair climber. They would burn about 861 calories during that 60-minute session (See References 2).
Impact of Intensity
While the calories-burned values provided by MayoClinic.com suggest that in most cases the stair climber burns more calories, they fail to account for the incline setting on the treadmill or the variations of speed possible on the stair climber. Both activities can be adjusted to make more challenging and thus burn more calories. According to Dr. Matthew Rhea's 2008 study for Free Motion Fitness, 30-year adults who walked at 3 mph at a 12% incline saw the same increase in heart rate as when they were running at 6 mph with no incline. This increase in heart rate is accommodated by an increase in calories burned, so those who walked at a 12% incline at 3 mph burn approximately the same number of calories as those who run on a flat surface at 6 mph (See References 4). Stair climbers allow you to increase the pace at which you're required to climb the stairs, with a faster pace obviously burning more calories. In addition, you could skip a stair when using the stair climber to increase intensity, which according to a 2012 study by Lewis G. Halsey, is effectively for significantly increasing calories burned (See References 5).
With both the treadmill and stair climber offering effective ways to burn calories, the best choice for you is the one that you feel most comfortable on which to kick up your workout intensity. A 1996 study published in the The Journal of the American Medical Association found that the treadmill and stair climber were the top two cardio machines for burning calories (See References 7). However, the American College of Sports Medicine stresses that the way to optimize how many calories you burn during your workout is to vary the intensity of your workout. If unable to maintain a high intensity for time, increasing the treadmill incline or the pace on the stair climber in short intervals will make large impacts on energy expenditure (See References 6, Pages 1). Also keep in mind that if you dislike one particular cardio machine, such as the stair climber, it's going to adversely affect how consistent you are with your workouts. Therefore, in that case, it would help to work out on the treadmill, as you'd end up getting more workouts in per week and thus burning more overall calories.