Most people don't have unlimited data plans for their smart phone, and are surprised to see very high data usage after just a few hours of streaming their wifi baby monitor. With HD video, you can go through a couple gigabytes of data in just a few hours, so keep that in mind. If your phone is connected to wifi that won't matter, but if you're using 3G or 4G LTE cellular service, you will definitely have slow video and tons of data usage.
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.