TreadmillPros - The treadmill is the leader in burning calories among cardio machines. It is also simple to use and does not require adjustments from person to person, so there is no fear of using the machine incorrectly. Many treadmills also have an incline or decline option for lower or higher difficulty. Treadmills help train the natural running motion, so it is an effective cardio machine for everyday life. It is also far more difficult to cheat and slack off on a treadmill.
Cons - The downside of the treadmill is that there is more impact on the joints than with other cardio machines. If you have not worked out in a while, it would be better to ease in to the other cardio machines to avoid soreness and too much stress on the joints.
EllipticalPros - Elliptical machines are also found in every gym and are extremely popular due to less impact on the joints. Many gym users also comment on the fun factor when using an elliptical, as quality elliptical machines create a smooth and flowing motion. Many elliptical machines also utilize arm motions which would be beneficial for a full body workout.
Cons - The elliptical does not burn as many calories as the treadmill. Also, if you are training for a sport that involves running or just running in general, it would generally be more beneficial to repeat the exact running form in order to gain muscle memory. Unfortunately, due to the comfort of the machine, it is also easier to just ease along without much intensity. Luckily, to combat this, many elliptical machines have resistance settings to further increase difficulty.
BikesPros - Stationary bikes (whether they are upright bikes or recumbent bikes) offer a complete cardio workout with the least impact on the joints. Fitness bikes are also great for people who are training for bike events because they can get used to the motion of pedaling. Many bikes are also equipped with televisions, which would allow for goers to catch up on their shows while still getting a quality workout.
Cons - To make best use of the stationary bike, users must adjust it to maximize the workout and avoid injury. Many people sit too low on the bike, which causes too much pressure on the knee. Your legs should be able to go through the full range of motions when pedaling on the bike.
Cons - Due to the constant standing motion, users with back issues may not want to use the stepmills or stairsteppers. Also if users clutch the railings of the stair climbers, there will only be minimal benefits. This machine also leads users to be more prone to accidents due to the climbing nature of it.
ConclusionThe main factor that you should be looking for when choosing a cardio machine is enjoyableness. If working out becomes a chore, then you're less likely to exert yourself and won't work out as long. Even if another cardio machine has a higher calories burned per hour ratio, if you don't enjoy the machine, you won't exert enough effort to burn those calories anyway. Each machine has its pros and cons, so the best technique is to try them all out and find out what you enjoy!