Both Treadmills and Rowing Machines are great for exercise and working out and improving your health.
But when it comes to burning calories the Treadmill is best as running on a Treadmill at a steep incline will burn more calories than using a rowing machine for the same amount of time.
However using a Rowing Machine can help build muscles and tone your body along with losing weight so for weight loss the Rowing machine is usually better.
But if you use both a Rowing Machine and a Treadmill for weight loss you'll get the benefit of both and lose weight faster.