A treadmill will focus only on the lower part of your body; while a roaring machine will give focus yours on the entire body. In this case, the roaring machine has the advantage of giving you a full-body workout. But if you consider the burning calories per hour then the treadmill will give you better performance.
You can use both machines for low impact exercise. On treadmill when you are walking then it will be low impact but if you want to jog or running then it becomes a high impact workout. And this high impact workout might harm your knees. On the other hand, a rowing machine will give you low impact always no matter how intensely you use it. And it is very helpful for your knees. So if you want a low impact workout and unable to control high impact workout anymore, then a rowing machine is the better option for you.
If your goal is to build muscles, then you should go for a rowing machine. Because rowing machine is more effective to build muscles as it works the muscles in both the upper and lower body at the same time. Where the treadmill only works on the lower body of your muscles.
Some people have the goal to reduce fat. If you are one of them then go for a treadmill. Because treadmill can reduce your fat faster than rowing machine.
These two machines have their own advantages and disadvantages. It is your choice which one you should prefer. Better you use both of them. And after using a few days you may able to understand which one is in your type and which one you should buy. So pick your desire one early and have a healthy workout.