I have owned the following piece of equipment:
1. Elliptical machine
2. Air powered Rower
3. Recumbent Bike
4. Pull up frame
Here is the thing, if you want to do any pull-ups, push ups, or any other sort of exercises that work the arms you can't use the rower to get your cardio exercise.
The elliptical is really easy to cheat on, I can do 10 miles on max resistance while watching a movie. I used to do 3 miles every day on it UNTIL my knees got really weak. My co worker said it was because I had the resistance up to high and the elliptical bike sets your feet too far apart (vs natural distance from running). Here is how you cheat. Set the resistance to at least 3 and then shift all of your weight onto the side of your body where the pedal is a little past the max distance from the floor. Your weight will push the pedal down and the elliptical's mechanics will push the opposite pedal to the max distance from the floor. All you're doing is basically shifting your weight from left to right and back. You'll work up a sweat eventually but it's not much of a real workout.
Pull up frames kick ass, but need a lot of room (height). Don't get the doorway ones because those will mess your door frame. It was the cheapest piece of equipment ($100) and I definitely got my money's worth. If you don;t have the space you can just do Hindu Pushups for an about equivalent workout.
The Recumbent Bike, I have owned 2. I first bought a cheap one branded Spartan or something like that from Sports Authority and it eventually broke. I replaced it with a Schwinn I got from Amazon. It's definitely worth the extra money and the best low impact cardio device for me since I do pull ups and hindu push ups to strengthen my upper body.