Treadmill or elliptical, that is the question. Which machine is better? Let's compare the two pieces and help you come to a conclusion.
Path of Motion
The treadmill will allow you to have that same natural feeling as walking or running, as it is not restrictive to a specific path of motion – you are actually walking or running like you would outside. Most ellipticals are on a fixed path so they dictate your stride movement and consequently do not feel quite the same as natural walking or running. This isn't bad it's just different. Some ellipticals have adjustable stride lengths to accommodate different people on the same machine.
Conclusion: If you're an avid walker or runner and prefer that specific feeling, a treadmill will most likely be the winner of this category. If you're going with an elliptical anyways, try and find one with adjustable stride length so you can partially customize the machine to fit yourself and everyone else who will use it.
Impact / Weight Bearing
The typical selling feature of an elliptical is the fact that it is "low-impact". What does this mean though? This means that every time you pick your foot off the ground/treadmill/etc. and then put it back on the ground as you step, you are creating a jarring force, or impact, on your body. Your body bears the weight of itself when you step down. That jarring is more intense if you were running compared to walking because your foot strikes the ground harder than if you were walking. Ellipticals are designed so your foot never leaves the pedals, therefore making it even lower impact, and less weight bearing, than walking. How does this translate to affect you? If you have health issues with your joints in your knees, hips, or back, as an example, a higher impact exercise puts extra pressure in areas that may be sore. Working out is hard enough without extra pain, so a lower impact exercise is better for you. However, if you can withstand some impact, studies have shown that appropriate impact, or weight bearing activities, help to strengthen bones and muscles. The last thing to note is that although it does not eliminate impact, most equipment comes with shock-absorbing designs, padded decks and pedals, and other cushioning to help minimize the harmful impact of cardio workouts.
Conclusion: For a lower impact workout the elliptical comes out on top. If you're going for a treadmill anyways, try turning up that incline and walking – it's lower impact than running but you'll burn similar calories.
Muscle Targeting
With all the features on treadmills and ellipticals, it is pretty easy to target your muscles on both. The key to this is using the incline feature. Some ellipticals don't have a ramp, or incline, so it becomes a little harder on those machines to target specific muscles. Adjusting the incline to be higher, or possibly even decline, will help target different leg muscles more specifically. This does depend slightly on your running/walking style, but in general, going up hill will target your calves and glutes more, while no incline will reach your quads. Some equipment, like the Spirit XE395 Elliptical has software to show you which muscles are most activated during your workout. The one drawback on this topic is that momentum of ellipticals will often take away from your muscle exertion.
Conclusion: Look for an incline feature to target muscles the best.
Higher Calorie Burn
Calorie burn is directly correlated to the intensity and effort you put into a workout. Bottom line, if you go hard, you'll burn more calories. Depending on the person will depend which piece of equipment will challenge a person more so choose what works best for you.
SUMMARY
You can analyze the pros and cons of each piece of equipment but the above is all food for thought and guidelines. The important thing is that you feel comfortable on the equipment you choose. It's highly recommended that you try out treadmills and ellipticals before you purchase one. Your investment will go further if you actually use your machine and you'll only use it if it's comfortable and doesn't cause you pain. Keep in mind that there are ways to mitigate most issues with equipment, for low impact on a treadmill turn up the incline, for a natural feel on an elliptical look for a variable stride length.