If you are doing weight-bearing activities (skiing, mountain running, mountaineering, etc.) then the weight-bearing training (stair stepping, treadmill, running) will be more effective because you will be working the muscles that keep you standing upright and support your weight. Rowing can give a great aerobic workout and good quad strength but you are sitting during the exercise. But, rowing can be a great recovery exercise, especially if you set low drag on the machine, because it’s low impact and gives your legs a high range of motion.