If you can, join a gym for a month or two and try out some different things. I have a treadmill at home and thought about buying something else, and was thinking a stationary bicycle or an elliptical. But I join the local university fitness center for 4 months every winter and discovered that I don't get a good enough workout on the bicycle and I hate the elliptical, but I absolutely love a stairstepper. I can get a great workout on the stairstepper with minimal impact (51 years old and failing knees) in as little as 20 minutes.
But if you're buying anything, consider:
Availability of levels to increase, decrease workout, and ease of changing those levels (you'll pay more for an electric treadmill incline changer, but it's a lot easier than manually doing it, which is what mine is.)
How important is a display? I track my workouts - time, mileage and calories mostly, but I also like to check my heart rate occasionally, so I need something with an electronic display with at least those features. (If it weren't for this requirement, I would be tempted to buy the air stepper that I have seen on infomercials - small, and easy to use and store.)
Do you want to be able to multi-task? Watch TV or read while working out? Think about a reading accessory or holder for the TV remote(s), and don't forget a cupholder for a water bottle.