The most important thing to look for in all kinds of baby monitors is audio quality. Regardless of whether you want a video-based baby monitor or not, you need clear audio so you can hear your baby properly. You'll also want one with sound activation so that you don't have to listen to white noise 90% of the time. With sound activation, you'll only hear the noises from your baby's room when there's something important to hear.
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.