The most reliable type of movement sensing product is the mattress pad option. This product is placed under the mattress, usually on a hard board and will only work with some types of mattresses. This unit relays messages to a nursery device that alerts parents on a parent device or with an alarm in the nursery (model dependent). The BabySense 7 is a good example of a sensor pad that works under your baby's mattress.
We tested each product we purchased in key metrics that allow them to function as expected, or provides an additional feature or benefit. Monitors act as something of a lifeline for parents, so it is important that they work well, have adequate range, provide useful images, and are easy to use. If a product doesn't work well enough to instill confidence, then it will fail to offer parents the one thing they really want, more sleep.
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.