Think about the size of your home and your daily routine when deciding which brand/model to buy. If you'll be making calls during nap time, for example, look for monitors with lights that let you know when your baby is awake. Of course you can accomplish the same thing by turning down the sound on a video monitor, but lights are more likely to catch your eye.
If your internet goes down, your baby monitor goes down. A wifi baby monitor uses your home's internet connection to communicate to servers, then to your app. So if your internet connection goes down, then you will no longer be able to stream video of your baby. That's a huge consideration to keep in mind because you never know when your internet will slow down or simply stop working for several minutes or hours, and you will be stuck without a working baby monitor. There are a few wifi baby monitors that will still communicate locally (within your home's network) when the internet goes down, such as the Lollipop or Nanit. They do this by switching from internet to local area network when an outage is detected, so you can have confidence that if you're connected to your home wifi you'll still see your baby.