Frequency used does matter
Better Choose a model with a frequency of 900 MHz or higher. This will reduce the possible interference
with other devices and increase clarity.Wireless preferred
A wireless receiver lets you access the signal both inside or outside the house. Also, having a belt clip helps you free up your hand when doing house-keeping.Video monitor & audio monitor
If you want to keep track your baby more, you can consider buying a video monitor and audio monitor. You can check your baby is sleeping peacefully or not via the video monitor withour leaving your bed. Audio monitor sends you signal when the baby cries.
Power source: batteries & A/C adapter
Look for a model that can be powered both by batteries and an A/C adapter. Batteries are helpful in case of a power failure and will let you use the unit outside.