Battery: We wanted a monitor battery that could last overnight, or at least eight hours, without being plugged in. We thought the ideal product would automatically cut off an idle display screen to conserve battery, work at least a few hours unplugged with the screen on, and recharge fairly efficiently. We made a rechargeable battery a requirement. We preferred units designed to connect to power via a standard USB connector, and looked for reports that the baby monitors could reliably charge, recharge, and hold a charge long-term—a disappointingly rare ability in baby monitors.
Some audio/video monitors filter out "normal" sounds and motions. The receiver is supposed to turn on only when your baby makes an unusual motion or sound, such as crying or rolling over when he's waking up from a nap. This feature is designed to extend battery life, although the receiver should be docked for overnight monitoring to keep the battery charged. We haven't tested any models with this feature, so we don't know how well it works.
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.