When you walk, your impact forces might be two times your body weight on a leg, while running can notch that up by a figure of 12. Impact forces are even higher for people carrying extra weight, of course. Repetitive exposure to high impact forces is considered the cause of many joint injuries, which is why the idea came along to help people work out without their feet ever catching air.
In that regard, elliptical trainers succeed. Studies show that elliptical trainers provide the same cardio workout as running, in terms of heart rate and oxygen consumption, while reducing the impact forces to that of walking. But impact forces are not all bad. You need them to increase your bone density (your body responds to the absorbing of shock by telling your bones to build new cells), and if you don't keep your bones dense when you are young, hormone loss when you are older will put you at greater risk of getting bent-over and shrunken.
But there's another issue here: Every time you absorb an impact you require your muscles to stabilize your joints, and practicing this makes you stronger at absorbing forces. Thus, impact forces are a particular conundrum, as getting too many may wear down your knees, but getting too few will mean you risk losing your eccentric (speed-slowing) strength, which in theory will make your joints more susceptible to injury. Confused? Then my work here is done.
Elliptical trainers are particularly good for anyone who has bad joints or is starting a training program carrying a few extra pounds. Which would be all of us. Treadmills, on the other hand, ask your body to get better at an activity it may actually be called upon to execute in the real world: running. And running entails absorbing the forces that come with living on planet Earth. But who's to say you have to be soI don't knowreasonable. Definitely try both machines out. Then buy whichever one seems fun enough that your very lucky sister will want to use it all the time.