What's Your Budget?
The most important factor in buying equipment is money. As a rule, you should spend as much as you can. If you only have a couple hundred dollars, is it worth it to buy the cheaper models? That depends. You can certainly find inexpensive models at discount department stores and they may work okay, but they won't always last and they may not be as sturdy as higher quality models.
Must Have Features
Features you want to look at include:
Stride Length. Some only offer 14" which would be too short for average-sized people so look for about 21".
Adjustable incline. This isn't necessary, but being able to adjust the ramps can add intensity to your workouts.
Smooth Motion. Make sure the pedals move smoothly and quietly - it shouldn't feel jerky.
Upper Body Option. You don't need arm handles, but it's a nice option for more intensity.
The Quiet Factor. You don't want your workout to sound like a freight train.
Adjustable resistance. Make sure you have a broad range of resistance for maximum benefits.
To keep your workout both safe and effective, make sure your elliptical trainer offers warm-up and cool-down periods, handrails that provide balance but don't get in the way, an easy-to-read console and, possibly, a heart rate monitor so you know how hard you're working. Also, look for a maintenance-free elliptical trainer and a good warranty (1 year for labor, 1-3 years for parts) just in case something happens. Oh, and make sure the dealer has trained staff to service your equipment.
This is the fun part. What kind of interactivity do you need? One to consider is a heart rate monitor. Try for the interactive version so that the machine will automatically adjust your workout to keep you in your zone. Decide how many pre-set programs you want (if any) and if you want to be able to create your own customized workouts as well. You may also want a machine with iFit.com capabilities--You hook up your machine to your computer and the website takes you through your workouts. The more features you choose, the more money you'll need to spend...but it's worth it if it'll help you stay on track.
Before you buy anything, visit several different retailers (preferably one that specializes in fitness equipment) and try several models. Wear your workout gear and use each machine for 10 minutes or so you get a feel for it. While testing things out, pay attention to the console; is it easy to see? Easy to use? Make sure the motion is smooth, not jerky, and that the handrails are convenient.
Finally, measure it to make sure you have enough space at home and don't forget to measure for ceiling height as well.