You might see monitors on the market with claims that they can track a baby's breathing or movements, but unless the unit is registered with the FDA, it's not a medical device. Consumer Reports hasn't tested this type of monitor. Talk with your pediatrician if you think your child has a condition that warrants medical monitoring. He or she can give you advice on the best devices.
You might prefer one that has lights as well as sound. All the audio monitors we rated had this feature. The Philips Avent DECT SCD510, which sells for about $120, has a series of small LED lights on the parent monitor. The louder the sound in your baby's room, the more lights go on, so you'll notice his crying even with the unit set on mute. Audio monitors are generally less expensive than audio/video models.
It really depends on what you feel most comfortable with. There are audio monitors that allow you to listen to any noise coming from the nursery, vital monitors that track sleep and breathing and video monitors that add sight to sound. Babylist parents overwhelmingly choose video monitors. The security of seeing what your child is up to—like if they’ve gotten tangled in their swaddle, pulled their diaper off or climbed out of the crib—can be worth the extra cost of a video monitor.